Animals are the most complex of all organisms (Figure 1). They are multicellular eukaryotes. Unlike plants and fungi, most animals are motile (they can move around), have amazing sensory ability, and are capable of very complex behaviours. These traits, along with a myriad of shapes, sizes, and ecological roles, create a truly remarkable kingdom, of which you are a member!
In this section we will focus on some particularly important and representative groups of animals. You will also study animals, with an emphasis on humans, in more detail in the Animals unit.
Figure 1 (a) The body structure of a sea slug, (b) the sensory ability of a damsel fly, (c) the ecological niche of a shield spider, and (d) the complex social behaviour of these lemurs hint at the incredible diversity within the Animal kingdom.
Why Animals Are Important
Humans have a special interest in animals—after all, we are animals too! The more we learn about the biology of animals, the more we learn about ourselves. As we improve our understanding of animal biology, we also improve our understanding of the origin and cause of human diseases and how best to keep our bodies and minds healthy.
We also depend on many other animal species for our own survival. We use many species of wild and domesticated animals as sources of food and other products. We depend on animals to pollinate important food crops, and we count on animals to play their important roles in maintaining the health of natural ecosystems. Humans also have a special affinity for their pets. Many people describe their pets as being part of their family.
But our interests in biology reach beyond finding out how other species benefit humans. Humans have always been fascinated by the remarkable diversity and characteristics of animals. Throughout history and across cultures, animals have been revered as symbols of power, beauty, freedom, and even peace. This admiration is shown in our deep concern for, and great efforts to protect, many endangered animal species, including giant pandas, elephants, rhinoceroses, polar bears, and Canada's peregrine falcons (Figure 2).
Figure 2 Capable of reaching speeds in excess of 300 km/h, the peregrine falcon is a symbol of power and agility. This species was once critically endangered, but it is making a comeback in Ontario.
Classification and Phylogeny
The common ancestor of all animals was likely a colonial, flagellated protist that lived at least 700 million years ago. Biologists have hypothesized that a spherical arrangement of cells in a colony might have become indented, forming a hollow cavity. This cavity would have helped the organism capture and digest food (Figure 3). This same basic arrangement and indenting process can be still observed in the embryonic development of animals today. Some cells in the colony may also have become specialized for feeding. This double-layered arrangement of cells with a lining of specialized digestive cells is very similar to that of sponges, the simplest of all modern animals. (See Figure 7, page 99.)
Figure 3 Scientists believe that animals evolved from colonial protists. The protists developed a hollow body cavity and specialized feeding cells. Over time, the colonies evolved into multicellular organisms with specialized tissues.
Image: Four pictures of how the colonial protest evolved. First as hollow body cavity with feeding cells around the outside, and over time evolving, changing shape, and forming an opening, the digestive cavity.
A key early innovation among animals was the development of nerves—specialized cells used to coordinate movements and sense changes in the environment. Animals in the Porifera phylum, such as modern sponges, are the only ones that do not have this key animal feature.
Another major division among animal phyla is between those with radial symmetry and those with bilateral symmetry. The bodies of animals with radial symmetry are regularly arranged around a central axis, like the spokes of a wheel. Jellyfish, for example, have radial symmetry. The bodies of animals with bilateral symmetry, such as humans, have left and right sides that are mirror images of each other. As you can see in Figure 4, bilaterally symmetrical animals also have different dorsal (upper) and ventral (lower) surfaces as well as an anterior (front) and posterior (back) end.
radial symmetry: symmetry around a central axis
bilateral symmetry: symmetry around a midline
Figure 4 (a) Radial symmetry and (b) bilateral symmetry
Image: (a) A plant; (b) A lobster, showing dorsal on top, ventral on bottom, anterior on front, and posterior on back.
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Bilaterally symmetrical animals are further divided into two major branches. The protostomes and deuterostomes are distinguished by different patterns of embryonic development. Humans belong to the deuterostome phylum Chordata. The Chordata are almost entirely vertebrates—animals with a dorsal backbone or notochord, including fish, amphibians, reptiles, birds, and mammals. All other animals are referred to as invertebrates.
protostome: an animal with bilateral symmetry; during embryonic development, the mouth forms before the anus
deuterostome: an animal with bilateral symmetry; during embryonic development, the anus forms before the mouth
vertebrate: an animal with a backbone or a notochord
notochord: a flexible rod found in some chordates; in most modern chordates it is replaced by vertebrae during embryonic development
invertebrate: an animal that does not have a backbone; the great majority of animal species are invertebrates
Figure 5 is a phylogenetic tree based on recent evidence from genetics and molecular biology. It illustrates one current hypothesis about the evolutionary relationships of 10 major phyla of animals. There are seven more animal phyla that are not shown on this tree, but they include relatively few species.
Figure 5 This phylogenetic tree of the Animal Kingdom is based on recent evidence from genetics and molecular biology. The tree shows 10 of the 17 animal phyla.
All animals are multicellular heterotrophs that use oxygen for aerobic respiration. Unlike plants and fungi, animal cells do not have cell walls. Their cell membranes are in direct contact with each other. Animals feed on plants, fungi, protists, and each other. A number of animals also have symbiotic relationships with autotrophs that provide a supplementary source of food.
Cell Specialization and Germ Layers
The nerve cells developed by the first animals are just one of the many specialized cell types in animals. These cells become specialized during embryonic development.
One way that animal phyla are distinguished from each other is by the number of germ layers their members have. Germ layers are the layers of cells in a developing embryo that give rise to specialized tissues. There are three germ layers: the ectoderm (ecto meaning "outer" and derm meaning "layer"), the endoderm (endo meaning "inner"), and the mesoderm (meso meaning "middle"). The ectoderm gives rise to the skin and nervous system. In some complex animals ectoderm cells produce shells, scales, feathers, hair, and nails. The endoderm forms the inner lining of the gut and, in some organisms, the respiratory system. The mesoderm gives rise to the circulatory, reproductive, excretory, and muscular systems.
germ layer: one of three layers that form during early embryonic development in most animals
Another feature that distinguishes animal phyla is whether or not the animals have a body cavity, or coelom, containing the internal organs (Figure 6). The coelom develops from the animal's mesoderm.
coelom: a body cavity present in some animals; contains the animal's internal organs
Figure 6 Most animals have one of two types of coelom.
Image: A diagram of a human inside midsection area, showing the gut, organs, coelom, and epidermis.
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The Simplest Invertebrates
The phyla Porifera and Cnidaria include the simplest invertebrate animals. The Porifera include about 8000 species of sponges. As illustrated in Figure 7, sponges have a simple body plan with flagellated cells, called choanocytes, lining a central cavity. The flagella create a continuous current of water that passes through pores in the body wall and exit through large openings at the top of the sponge. Food particles entering with water are captured by the choanocytes. Porifera are sessile as adults and range in size from 1 cm to 2 m across (Figure 8(a)). Most sponges are hermaphrodites that release sperm into the surrounding water. Their eggs remain within their bodies and are fertilized by other sperm that are drawn in with water. The fertilized eggs develop into ciliated larvae that swim or crawl in search of a suitable location to settle and grow.
Figure 7 Sponges have a simple body plan with two main layers. The inner layer consists of specialized feeding cells called choanocytes that are similar in structure to some protists.
Image: A sponge, showing its body plan. The choanocytes is the inner layer, osculum points to the upper inside section, dpongocoel ponts to the inner mid section. Water is shown entering throught the pores and exiting out the top of sponge.
Figure 8 (a) Porifera like this tube sponge are considered the simplest animals.
Most sponges are marine, but there are more than 100 freshwater species, including some that are common in Ontario. Freshwater sponges are often mistaken for plants because they contain symbiotic blue-green algae, or cyanobacteria. Marine sponges can be very abundant, and they play important ecological roles as food and shelter for many other organisms—for example, large sponges can harbour thousands of shrimp. Sponges are also commercially valuable and are threatened by overharvesting in some regions.
The Cnidaria include close to 9000 species of hydras, anemones, jellyfish, and coral animals (Figure 8(b) and (c)). They do not have a mesoderm and exhibit radial symmetry. They are the simplest animals with specialized nerve, muscle, digestive, and reproductive tissues. All cnidarians have tentacles with stinging cells that contain nematocysts, which are capsules containing a barbed, thread-like tube that delivers a paralyzing sting when propelled into attackers and prey.
Figure 8 (b) Some cnidarians, like this giant Nomura jellyfish, can measure 2 m across and exceed 200 kg in weight. Some are free swimming, while others, like (c) this anemone, are sessile.
nematocyst: a capsule within specialized cells of cnidarians containing toxins that can be propelled toward attackers and prey, causing them to become paralyzed; also called a stinging cell
Coral animals are cnidarians that produce external skeletons made of calcium carbonate. Over time, these skeletons can accumulate, and large communities of coral animals can create gigantic coral reefs. These reefs form complex structures and provide habitat for thousands of other marine species. They are considered the most biologically diverse of all aquatic ecosystems (Figure 9).
Figure 9 Communities of coral animals can build complex reefs that are home to many other species.
Image: A colorful underwater reef and several fish swimming around.
Although the status of many cnidarians is not known, 236 species are considered at risk, mostly due to pollution and habitat destruction. The effects of greenhouse gases and climate change on water chemistry and temperatures may prove to be an even greater problem. Today, most of the world's coral reefs are seriously threatened.
A local species of cnidarian is the freshwater jellyfish Craspedacusta sowerbyi. This jellyfish is harmless to humans, but it is an invasive species that is spreading around the world. It was first reported in Ontario in 1980.
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The Protostome Invertebrates
The protostome invertebrates include six major animal phyla and the vast majority of all animal species (Table 1). They occupy almost every environment and in many cases are extremely abundant.
Table 1 The Six Major Phyla of Protostome Animals
Phylum (common name): Arthropoda (arthropods) Examples (Animals shown in the photos are bolded*): insects*, spiders, mites, ticks, millipedes, centipedes, scorpions, daphnia, crabs, lobsters, shrimps, barnacles Key features: - They have segmented bodies with jointed appendages. - They have complex sensory systems including antennae. - Their hard outer skeletons contain chitin. - They have highly variable body shapes. - They have complete digestive, excretory, and circulatory systems. - Their respiratory systems use gills or internal airways. - They range in size from 0.1 mm to 1 m in length. - They are extremely diverse and play vital roles in all aquatic and terrestrial ecosystems, often as the primary consumers. - Insects are the only invertebrates capable of flight. - About 1 100 000 species have been described (greater than 75 classes)
Phylum (common name): Nematoda (roundworms) Examples (Animals shown in the photos are bolded*): pinworms, dog heartworms* Key features: - They have unsegmented cylindrical bodies with complete digestive tracts. - Many are important parasites of other animals. - Some females can produce more than 100 000 eggs each day. - They live in very large numbers in the soil and aquatic sediments. - They range in size from 2 mm to 2 m in length. - About 20 000 species have been described (2 classes).
Phylum (common name): Annelida (segmented worms) Examples (Animals shown in the photos are bolded*): earthworm, feather-duster worms* Key features: - Their bodies and most of their internal organs are segmented. - They have complete digestive systems. - Gas is exchanged through the skin, gills, or other specialized body parts. - Most have bristles on their outer surface to help with movement. - Many are marine. - They range in size from 0.5 mm to 3 m in length. - About 14 000 species have been described (7 classes).
Phylum (common name): Mollusca (mollusks) Examples (Animals shown in the photos are bolded*): snails*, clams, octopuses, squid Key features: - They have three main unsegmented body parts: a foot, a visceral mass, and a mantle that secretes a shell. - Almost all have specialized file-like radula used for scraping and boring. - In some species the shell is reduced or absent. - They have complete digestive systems, circulatory systems, and gills. - They range in size from less than 1 cm to 20 m in length. - About 120 000 species have been described (8 classes).
Phylum (common name): Rotifera (rotifers) Examples (Animals shown in the photos are bolded*): rotifers* Key features: - They are small aquatic animals (less than 2 mm long). - They use cilia to direct food into their mouths. - Most live in fresh water. - They have no respiratory or circulatory system. - They are very important consumers in many aquatic food webs. - About 1800 species have been described (3 classes).
Phylum (common name): Platy- helminthes (flatworms) Examples (Animals shown in the photos are bolded*): tapeworms*, liver flukes Key features: - They are flattened, unsegmented worms. - Most have a digestive cavity with a single opening. - They do not have coeloms. - They range in size from 1 mm to 5 m in length. - Many are important parasites of other animals. - They have no circulatory or respiratory system. - About 14 000 species have been described (4 classes).
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Body Plans and Life Cycles
The body plans and life cycles of protostomes are extraordinarily diverse. Figure 10 shows a representative body plan from the most biologically successful class of organisms—Phylum Arthropoda, Class Insecta.
Figure 10 There are more known species of insects than of any other class of organisms on Earth. Their success is due in large part to an efficient body plan that includes nervous, digestive, respiratory, excretory systems and reproductive systems; advanced sensory structures; and the ability to fly.
Images: The external anatomy of a grasshopper: listing its small legs in front, and large legs in back, mouth parts, compound eye, antenna, head, thorax, wings, the hearing organ on front side under wings, and abdomen; Internal anatomy of a female grasshopper: Brain, mouth, dorsal blood vessel, heart, ovary, malpighian tubules, digestive system, and ventral nerve cord with ganglia.
Other characteristic body features of protostomes include the powerful claws of crabs and lobsters, the tentacles of octopuses and squid, the ornate shells of snails and clams, and the large, colourful wings of butterflies.
Many animals, including insects, have a life cycle that involves a number of distinct larval stages. In these life cycles, the larval stages are sometimes unrecognizable when compared to the adult. Figure 11 shows examples of four larvae.
Figure 11 The larval stages of an (a) octopus, (b) snail, (c) butterfly, and (d) mosquito.
Human-Protostome Interactions
The protostomes include many species that are of great consequence to humans and interact with us in direct ways. They affect our food supply, our health, and our economy.
COMPETITORS
Many protostomes compete with humans for food. Insects are some of the best known competitors. Virtually every food crop has a number of important insect pests. For example, important pests of Ontario corn crops include at least 13 different species of insect and one species of slug (Phylum Mollusca) (Figure 12).
Figure 12 Tomato hornworms are common pests in Ontario. These caterpillars feed on tomato plants, including the fruit.
Food losses due to such competitors are significant, and the cost of controlling these pests can be a major factor in the cost of food production. Most crops are planted in large monocultures. This, along with the ability of many insects to reproduce quickly, allows insect populations to grow very quickly. Pesticides can keep pest populations in check, but some insects are becoming resistant to pesticides. Pesticides can also damage the environment and pose risks to human health. Organic and alternative farming methods can eliminate or reduce the use of synthetic pesticides. They rely instead on the presence of natural or introduced predatory insects to control pests.
Insects are also serious pests of other commercially valuable plants including trees and cotton. The spruce budworm, gypsy moth, Asian long-horned beetle, and mountain pine beetle are examples of insects that have caused significant damage to forests.
PATHOGENS AND VECTORS
Humans suffer from many parasitic diseases. Some are caused by protists, but others are caused by animals—particularly species of nematodes, tapeworms, and flukes. Various nematode parasites are capable of infecting many body tissues. Adult tapeworms inhabit intestines and pass their fertilized eggs out with feces (Figure 13). Parasitic flukes often live in lungs or blood vessels. One blood fluke that normally infects waterfowl is responsible for "swimmer's itch."
In addition to causing disease, many biting animals are vectors—they spread disease. For example, mosquitoes spread malaria, yellow fever, and West Nile virus from person to person. Ticks and fleas spread Lyme disease and the plague. Controlling the spread of these diseases involves controlling the animal vectors.
Figure 13 (a) A tapeworm has a specialized head, or scolex. (b) Some tapeworms have heads with barbs and suckers for attaching to the wall of the intestine.
FOOD AND ECONOMIC BENEFITS
Many protostomes benefit humans. We eat many of these animals, including mollusks such as clams and scallops, crustaceans such as shrimp and lobsters, and the honey produced by bees. We use oysters for food and as a source of pearls (Figure 14). Coral reefs are used in some countries as a source of minerals for cement production, and silk is a very valuable fibre produced by caterpillars of the silkworm moth. Perhaps the greatest direct value we receive from protostomes is the pollination of food crops by insects. If plants are not pollinated, they do not produce seeds, and many do not produce fruit.
Figure 14 Some oysters secrete a protective hard coating around a foreign particle, such as a grain of sand, forming a pearl.
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The Deuterostomes
The deuterostomes are a much less diverse but much more familiar collection of animals. This branch of the animal phylogenetic tree includes two main groups: the echinoderms (starfish, sea urchins, sea cucumbers, and their closest relatives) and the chordates (vertebrates and their closest relatives).
The Echinoderms
The 6500 species of echinoderms are the only invertebrates that exhibit the same pattern of early embryonic development as the vertebrates. The immature stages of echinoderms are free swimming and bilaterally symmetrical. As they grow, they develop a radially symmetrical pattern around five or more arms (Figure 15(a)).
Adult echinoderms have a unique body plan. They have a complete digestive system, a simple circulatory system, and no respiratory or excretory system. They do not have a head region, and their nervous system circles the mouth and extends into the arms. Echinoderms move using a water-filled vascular system that uses the principles of hydraulics. By pumping water with muscle action and opening and closing valves within the system, echinoderms can control the movements of their arms and tube feet (Figure 15(b)). This system also allows them to easily generate considerable force. Some starfish use this ability to pull apart large clams and feed on them.
Figure 15 (a) Echinoderms such as this brittle star are bilaterally symmetrical as larvae but radially symmetrical as adults. (b) Echinoderms move using a system of water-filled canals that control their arms and tube feet.
The Chordates
The chordates include fish, amphibians, mammals, reptiles, and birds. Although the birds are discussed here as a separate class, they are now considered a recently evolved group of reptiles.
The chordates include the most complex living organisms. They are thought to have evolved from bilaterally symmetrical ancestors with segmented bodies. These primitive chordates had gill slits, a dorsal nerve cord, and a flexible rod called a notochord. The notochord acted as an anchor for attaching the body wall muscles (Figure 16). These features probably enhanced the chordate's ability to move. In primitive chordates and many modern chordates the gill slits are used for filter feeding.
Figure 16 These are the distinguishing features of the common ancestor of all chordates. The simplest living chordates still retain this body form.
Image: The features of the chordates show the mouth at front, the dorsal hollow nerve cord inside top front, the notochord inside middle under dorsal hollow nerve cord, segmented body wall muscles surrounding inside parts, the anus exiting out back bottom, and the figestive tract which runs between the mouth and anus.
Over time a number of other important features evolved, including vertebrae, paired appendages, a bony skeleton, and more highly developed brain and sensory systems. Internal supporting skeletons enabled chordates to develop large body sizes—most vertebrates are many times larger than the vast majority of invertebrates. The blue whale, a vertebrate, is believed to be the largest animal that ever lived.
A key development that enabled some of the vertebrates to conquer land was the evolution of a waterproof amniotic egg. An amniotic egg has specialized membranes and an outer shell that make it resistant to water loss. Mammals, including humans, retain amniotic membranes around the developing fetus. Only a few species of mammals, including the platypus and the echidna, still lay shelled eggs.
amniotic egg: an egg with an outer leathery or hard shell and specialized internal membranes that protect and nourish the embryo
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The phylogenetic tree in Figure 17 indicates major features that arose in the evolutionary history of the chordates. The birds are shown as a distinct group, although they are now considered to belong to the reptile Glade.
Figure 17 The phylogenetic tree for the deuterostomes
Most chordates are vertebrates—their dorsal nerve cord is enclosed within a spinal column and cranium, or skull. The spinal column is divided into separate segments called vertebrae. Except for the earliest fish, all vertebrates have a well-defined head containing a cranium that surrounds and protects an enlarged brain. Unlike the external skeleton of arthropods, which the animal must shed in order to grow, the internal skeleton of vertebrates can grow along with the animal's body. The enlarged and protected brains and eyes, ears, and noses characteristic of vertebrates provide advanced sensory capability and enable coordinated behaviours and motion.
The majority of vertebrate species are fish. Their vertebrate skeletons and large brains enabled them to become powerful and agile swimmers, far surpassing the swimming capabilities of any invertebrates. Over time some fish evolved paired appendages, which gave rise to bony limbs. These fish were to be the ancestors of early amphibians and all land vertebrates.
Vertebrates evolved a number of key features that enabled them to conquer the land. Beginning with the limbs that allowed early amphibians to support their bodies on land, early reptiles evolved protective outer skin, enhanced lungs and circulatory systems, internal fertilization, and eggs with outer shells (Figure 18).
Figure 18 This leatherback sea turtle hatchling will scurry across the sand to reach the ocean, where it will spend most of its life.
Image: A leatherback sea turtle hatching from its egg.
Although greatly outnumbered by invertebrates, vertebrates are nonetheless a diverse and dominant group of animals. Vertebrates have the most advanced organ systems and the most complex behaviours of all organisms. These features, along with their large size, enable them to inhabit many aquatic and terrestrial environments. In addition, birds and mammals can maintain warm body temperatures. This lets them remain active in very cold environments when all other forms of life are dormant.
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Table 2 highlights the major taxonomic groups within the vertebrates. Notice that the first three groups are often collectively referred to as "fish," but they are actually quite distinct groups.
Table 2 The Vertebrates
Class: Agnathans (jawless fishes) Examples (Animals shown in the photos are bolded*.): lamprey*, hagfish Key features: - Their skeletons are made of cartilage, they have no jaws, and a notochord is present in adults. - They have gill slits but no paired appendages. - Hagfish are scavengers. - Many lampreys are parasites of fish. - Lampreys have larval forms that are filter feeders. - There are marine and freshwater species. - About 60 species have been described.
Class: Chondrichthyes (cartilaginous fishes) Examples (Animals shown in the photos are bolded*.): sharks*, rays, skates Key features: - Their skeletons are made of cartilage. - They have jaws, vertebrae, gill slits, and paired appendages. - Their fins are often thick. - Many are predators of other fish. - Reproduction uses internal fertilization. - Most species are marine. - About 850 species have been described.
Class: Actinopterygii (bony fishes) Examples (Animals shown in the photos are bolded*.): most common fish—bass, trout*, tuna, salmon Key features: - Their skeletons are bony. - Most have a swim bladder—a gas-filled organ used to control buoyancy. - Their fins are often membrane-like with stiff "rays” for support. - Most species use external fertilization. - There are marine and freshwater species. - About 24 000 species have been described.
Class: Amphibia (amphibians) Examples (Animals shown in the photos are bolded*.): frogs, salamanders*, caecilians Key features: - Most have an aquatic larval stage with gills. - Adults are tetrapods—having four limbs adapted for moving on land (except caecilians). - Most species use external fertilization. - They breathe through their lungs and/or skin. - About 5000 species have been described.
Class: Reptilia (reptiles and birds*) *Birds are described separately. Examples (Animals shown in the photos are bolded*.): snakes, lizards*, crocodilians Key features: - Most are terrestrial tetrapods with dry scaly skin. - They breathe with lungs. - They use internal fertilization. - They have amniotic eggs with soft shells. - Turtles lack teeth and have a bony shell. Aquatic species have webbed feet. - Turtles are the most distantly related group within the reptile clade. - About 7300 species have been described.
Class: Ayes (birds) Examples (Animals shown in the photos are bolded*.): birds Key features: - They are tetrapods with forelimbs modified as wings. - Most species are capable of flight. - They have feathers. - They are warm-blooded (endothermic). - They have large brains and acute vision. - They use internal fertilization. - They lay hard-shelled amniotic eggs. - About 9700 species have been described.
Class: Mammalia (mammals) Examples (Animals shown in the photos are bolded*.): mammals Key features: - They are tetrapods and have hair. - They nurse their young with milk produced in mammary glands. - They are warm-blooded (endothermic). - They have large brains and acute vision and sense of smell. - They use internal fertilization. - Most give birth to live young; a few species produce amniotic eggs. - About 4500 species have been described.
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VERTEBRATE DIVERSITY AND THE WEB OF LIFE
Although greatly outnumbered by invertebrates, vertebrates are a diverse and important group of animals. When you hear the word "fish," "snake," "frog," "bird," or "mammal" you may envision "typical" examples. You might even think that the members of each of these groups are quite similar. How different is one kind of fish from another kind of fish, or one kind of snake from another? Is there really that much difference between frog or bird species? Do all mammals not have very similar characteristics?
As you have learned throughout this unit, species are grouped according to many shared features, but each species possesses a unique set of features that sets it apart from every other species on Earth. The examples listed in Table 3 are just a tiny sampling of the countless unusual and extraordinarily diverse vertebrates.
Table 3 Highlighting Vertebrate Diversity
Feature: Size Examples: - Blue whales can have a mass of more than 150 000 kg. Their hearts alone can have a mass of 600 kg. They give birth to a single calf with a mass of more than 2500 kg. The calf drinks 400 L of milk each day! - The smallest bird is the bee hummingbird of Cuba. It has a mass of just 1.8 g, the same as a dime. This extraordinarily small flying machine is capable of beating its wings 80 times per second and visiting more than 1000 flowers in a single day.
Feature: Sensory ability Examples: - Echolocating bats can detect an echo off an object as small as a mosquito. They can also produce and detect sounds with frequencies far beyond the range of human hearing. - Pit viper snakes have the ability to detect infrared radiation. Special pits on the sides of their heads detect this thermal energy, which is given off by warm-blooded mammals and birds.
Feature: Behaviour Examples: - The longest migration of any animal is that of the arctic tern. Their annual zigzagging return trip can cover a distance of over 71 000 km! - Arctic ground squirrels, in contrast, hibernate over the winter. Arctic winters are so long that these ground squirrels spend more than 80 % of their lives asleep!
Feature: Life cycles Examples: - Finding a mate in the deep ocean is extremely difficult. So when the tiny male angler fish finds one, he bites onto the body of the female and remains attached to her for the rest of his life. The body of the female grows into the mouth of the male, and their circulatory systems fuse. When the female is releasing her eggs, hormones in her blood travel into the body of the male and cause it to release sperm. - Females of the recently extinct gastric brooding frog swallowed their fertilized eggs. The eggs hatched in their mother's stomach, and the tadpoles developed there fully before climbing up her throat and hopping away!
Humans have a special interest in mammals and birds. This is partly because of their fascinating diversity of forms and behaviours, but it is no doubt also because we are vertebrates. This may explain why humans are much more likely to be concerned about threats to these species than they are about threats to invertebrate species or species in other kingdoms. Many people join organizations to help protect endangered mammals and birds, but few organizations focus on saving invertebrates, fungi, or plants.
Fortunately, most current efforts to protect the environment and the diversity of life on Earth focus on protecting habitats and entire ecosystems. Concern over species such as panda bears, polar bears, and gorillas captures public interest and media attention (Figure 19). If this interest is used to protect the ecosystems to which these wonderful species belong, the entire web of life within those ecosystems benefits. In this way, our interest in the vertebrates may be our most powerful means of protecting the diversity of life.
Figure 19 Roots and Shoots and the Jane Goodall Foundation promote environmental awareness and the protection of gorillas. Jane Goodall began her monumental work with wild gorillas in 1960.
Image: A mother gorilla sitting in the grass holding her sleeping baby gorilla in her arms.
WEB LINK
To learn about environmental organizations and how you can get involved, GO TO NELSON SCIENCE
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3.3 Summary
- Animals are an extremely large and diverse group of heterotrophic multicellular eukaryotes.
- Almost all animals are motile and have nerves that coordinate body movements and process sensory information.
- Animals are very diverse in their morphology and life cycles.
- Most animals develop from an embryo with three germ layers: the endoderm, the mesoderm, and the ectoderm.
- The simplest animals are the Porifera and the Cnidaria.
- Most animals are protostomes: arthropods, nematodes, mollusks, annelids, rotifers, or platyhelminthes.
- Humans have many important interactions with protostomes.
- The chordates are the most complex of all organisms and include the vertebrates: fish, amphibians, mammals, reptiles, and birds.
- The evolution of a bony skeleton, paired limbs, waterproof skin, and an amniotic egg enabled vertebrates to survive on land.
- The diversity of animals is threatened by many human activities, including habitat destruction, pollution, and climate change, but our affinity for animals motivates us to protect them and their habitats.
3.3 Questions
Key
K/U: Knowledge and Understanding T/I: Thinking and Investigation C: Communication A: Application
1. Animals, plants, and fungi are all multicellular eukaryotes. What unifying characteristics distinguish animals from members of these other two kingdoms? K/U
4. Match the following traits to their animal group below. The traits are present in more than one group. K/U C
5. Which body system does each of the three germ ayers give rise to? K/U
7. State two key characteristics and two examples for each of the following phyla: K/U C (a) Mollusca (b) Nematoda (c) Arthropoda (d) Annelida
8. The platyhelminthes are considered simple but important animals. Justify this description. K/U C
11. What five major evolutionary advances are shared by all mammals, reptiles, and birds? Explain the benefit of each advance. K/U C
12. What special feature of birds and mammals has enhanced their ability to survive in cold climates? K/U
13. Marine ecosystems are vulnerable to human actions but often remain hidden from our view. The coral reefs in the Caribbean are subject to many human impacts and are in a general state of decline. Use the Internet and other sources to learn about the status of the Caribbean reefs. [GO TO NELSON SCIENCE] T/I (a) What are the major human factors that have caused their decline? (b) What is a lionfish? How is it influencing these reefs? (c) How is climate change expected to affect these reefs?
Animals are the most complex of all organisms (Figure 1). They are multicellular eukaryotes. Unlike plants and fungi, most animals are motile (they can move around), have amazing sensory ability, and are capable of very complex behaviours. These traits, along with a myriad of shapes, sizes, and ecological roles, create a truly remarkable kingdom, of which you are a member!
In this section we will focus on some particularly important and representative groups of animals. You will also study animals, with an emphasis on humans, in more detail in the Animals unit.
Figure 1 (a) The body structure of a sea slug, (b) the sensory ability of a damsel fly, (c) the ecological niche of a shield spider, and (d) the complex social behaviour of these lemurs hint at the incredible diversity within the Animal kingdom.
Why Animals Are Important
Humans have a special interest in animals—after all, we are animals too! The more we learn about the biology of animals, the more we learn about ourselves. As we improve our understanding of animal biology, we also improve our understanding of the origin and cause of human diseases and how best to keep our bodies and minds healthy.
We also depend on many other animal species for our own survival. We use many species of wild and domesticated animals as sources of food and other products. We depend on animals to pollinate important food crops, and we count on animals to play their important roles in maintaining the health of natural ecosystems. Humans also have a special affinity for their pets. Many people describe their pets as being part of their family.
But our interests in biology reach beyond finding out how other species benefit humans. Humans have always been fascinated by the remarkable diversity and characteristics of animals. Throughout history and across cultures, animals have been revered as symbols of power, beauty, freedom, and even peace. This admiration is shown in our deep concern for, and great efforts to protect, many endangered animal species, including giant pandas, elephants, rhinoceroses, polar bears, and Canada's peregrine falcons (Figure 2).
Figure 2 Capable of reaching speeds in excess of 300 km/h, the peregrine falcon is a symbol of power and agility. This species was once critically endangered, but it is making a comeback in Ontario.
Classification and Phylogeny
The common ancestor of all animals was likely a colonial, flagellated protist that lived at least 700 million years ago. Biologists have hypothesized that a spherical arrangement of cells in a colony might have become indented, forming a hollow cavity. This cavity would have helped the organism capture and digest food (Figure 3). This same basic arrangement and indenting process can be still observed in the embryonic development of animals today. Some cells in the colony may also have become specialized for feeding. This double-layered arrangement of cells with a lining of specialized digestive cells is very similar to that of sponges, the simplest of all modern animals. (See Figure 7, page 99.)
Figure 3 Scientists believe that animals evolved from colonial protists. The protists developed a hollow body cavity and specialized feeding cells. Over time, the colonies evolved into multicellular organisms with specialized tissues.
Image: Four pictures of how the colonial protest evolved. First as hollow body cavity with feeding cells around the outside, and over time evolving, changing shape, and forming an opening, the digestive cavity.
A key early innovation among animals was the development of nerves—specialized cells used to coordinate movements and sense changes in the environment. Animals in the Porifera phylum, such as modern sponges, are the only ones that do not have this key animal feature.
Another major division among animal phyla is between those with radial symmetry and those with bilateral symmetry. The bodies of animals with radial symmetry are regularly arranged around a central axis, like the spokes of a wheel. Jellyfish, for example, have radial symmetry. The bodies of animals with bilateral symmetry, such as humans, have left and right sides that are mirror images of each other. As you can see in Figure 4, bilaterally symmetrical animals also have different dorsal (upper) and ventral (lower) surfaces as well as an anterior (front) and posterior (back) end.
radial symmetry: symmetry around a central axis
bilateral symmetry: symmetry around a midline
Figure 4 (a) Radial symmetry and (b) bilateral symmetry
Image: (a) A plant; (b) A lobster, showing dorsal on top, ventral on bottom, anterior on front, and posterior on back.
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Bilaterally symmetrical animals are further divided into two major branches. The protostomes and deuterostomes are distinguished by different patterns of embryonic development. Humans belong to the deuterostome phylum Chordata. The Chordata are almost entirely vertebrates—animals with a dorsal backbone or notochord, including fish, amphibians, reptiles, birds, and mammals. All other animals are referred to as invertebrates.
protostome: an animal with bilateral symmetry; during embryonic development, the mouth forms before the anus
deuterostome: an animal with bilateral symmetry; during embryonic development, the anus forms before the mouth
vertebrate: an animal with a backbone or a notochord
notochord: a flexible rod found in some chordates; in most modern chordates it is replaced by vertebrae during embryonic development
invertebrate: an animal that does not have a backbone; the great majority of animal species are invertebrates
Figure 5 is a phylogenetic tree based on recent evidence from genetics and molecular biology. It illustrates one current hypothesis about the evolutionary relationships of 10 major phyla of animals. There are seven more animal phyla that are not shown on this tree, but they include relatively few species.
Figure 5 This phylogenetic tree of the Animal Kingdom is based on recent evidence from genetics and molecular biology. The tree shows 10 of the 17 animal phyla.
Image: A phylogenetic tree.
colonial choanoflagellate ancestor
- (no nerves) Porifera
- (nerves) (radial symmetry) Cnidaria
- (bilateral symmetry) (protostomes) Platyhelminthes, Rotidera
- (bilateral symmetry) (protostomes) Mollusca, Annelida
- (bilateral symmetry) (protostomes) Nematoda, Arthropda
- (bilateral symmetry) (deuterostomes) Echinodermata, Chordata
Characteristics
All animals are multicellular heterotrophs that use oxygen for aerobic respiration. Unlike plants and fungi, animal cells do not have cell walls. Their cell membranes are in direct contact with each other. Animals feed on plants, fungi, protists, and each other. A number of animals also have symbiotic relationships with autotrophs that provide a supplementary source of food.
Cell Specialization and Germ Layers
The nerve cells developed by the first animals are just one of the many specialized cell types in animals. These cells become specialized during embryonic development.
One way that animal phyla are distinguished from each other is by the number of germ layers their members have. Germ layers are the layers of cells in a developing embryo that give rise to specialized tissues. There are three germ layers: the ectoderm (ecto meaning "outer" and derm meaning "layer"), the endoderm (endo meaning "inner"), and the mesoderm (meso meaning "middle"). The ectoderm gives rise to the skin and nervous system. In some complex animals ectoderm cells produce shells, scales, feathers, hair, and nails. The endoderm forms the inner lining of the gut and, in some organisms, the respiratory system. The mesoderm gives rise to the circulatory, reproductive, excretory, and muscular systems.
germ layer: one of three layers that form during early embryonic development in most animals
Another feature that distinguishes animal phyla is whether or not the animals have a body cavity, or coelom, containing the internal organs (Figure 6). The coelom develops from the animal's mesoderm.
coelom: a body cavity present in some animals; contains the animal's internal organs
Figure 6 Most animals have one of two types of coelom.
Image: A diagram of a human inside midsection area, showing the gut, organs, coelom, and epidermis.
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The Simplest Invertebrates
The phyla Porifera and Cnidaria include the simplest invertebrate animals. The Porifera include about 8000 species of sponges. As illustrated in Figure 7, sponges have a simple body plan with flagellated cells, called choanocytes, lining a central cavity. The flagella create a continuous current of water that passes through pores in the body wall and exit through large openings at the top of the sponge. Food particles entering with water are captured by the choanocytes. Porifera are sessile as adults and range in size from 1 cm to 2 m across (Figure 8(a)). Most sponges are hermaphrodites that release sperm into the surrounding water. Their eggs remain within their bodies and are fertilized by other sperm that are drawn in with water. The fertilized eggs develop into ciliated larvae that swim or crawl in search of a suitable location to settle and grow.
Figure 7 Sponges have a simple body plan with two main layers. The inner layer consists of specialized feeding cells called choanocytes that are similar in structure to some protists.
Image: A sponge, showing its body plan. The choanocytes is the inner layer, osculum points to the upper inside section, dpongocoel ponts to the inner mid section. Water is shown entering throught the pores and exiting out the top of sponge.
Figure 8 (a) Porifera like this tube sponge are considered the simplest animals.
Most sponges are marine, but there are more than 100 freshwater species, including some that are common in Ontario. Freshwater sponges are often mistaken for plants because they contain symbiotic blue-green algae, or cyanobacteria. Marine sponges can be very abundant, and they play important ecological roles as food and shelter for many other organisms—for example, large sponges can harbour thousands of shrimp. Sponges are also commercially valuable and are threatened by overharvesting in some regions.
The Cnidaria include close to 9000 species of hydras, anemones, jellyfish, and coral animals (Figure 8(b) and (c)). They do not have a mesoderm and exhibit radial symmetry. They are the simplest animals with specialized nerve, muscle, digestive, and reproductive tissues. All cnidarians have tentacles with stinging cells that contain nematocysts, which are capsules containing a barbed, thread-like tube that delivers a paralyzing sting when propelled into attackers and prey.
Figure 8 (b) Some cnidarians, like this giant Nomura jellyfish, can measure 2 m across and exceed 200 kg in weight. Some are free swimming, while others, like (c) this anemone, are sessile.
nematocyst: a capsule within specialized cells of cnidarians containing toxins that can be propelled toward attackers and prey, causing them to become paralyzed; also called a stinging cell
Coral animals are cnidarians that produce external skeletons made of calcium carbonate. Over time, these skeletons can accumulate, and large communities of coral animals can create gigantic coral reefs. These reefs form complex structures and provide habitat for thousands of other marine species. They are considered the most biologically diverse of all aquatic ecosystems (Figure 9).
Figure 9 Communities of coral animals can build complex reefs that are home to many other species.
Image: A colorful underwater reef and several fish swimming around.
Although the status of many cnidarians is not known, 236 species are considered at risk, mostly due to pollution and habitat destruction. The effects of greenhouse gases and climate change on water chemistry and temperatures may prove to be an even greater problem. Today, most of the world's coral reefs are seriously threatened.
A local species of cnidarian is the freshwater jellyfish Craspedacusta sowerbyi. This jellyfish is harmless to humans, but it is an invasive species that is spreading around the world. It was first reported in Ontario in 1980.
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The Protostome Invertebrates
The protostome invertebrates include six major animal phyla and the vast majority of all animal species (Table 1). They occupy almost every environment and in many cases are extremely abundant.
Table 1 The Six Major Phyla of Protostome Animals
Phylum (common name): Arthropoda (arthropods)
Examples (Animals shown in the photos are bolded*): insects*, spiders, mites, ticks, millipedes, centipedes, scorpions, daphnia, crabs, lobsters, shrimps, barnacles
Key features:
- They have segmented bodies with jointed appendages.
- They have complex sensory systems including antennae.
- Their hard outer skeletons contain chitin.
- They have highly variable body shapes.
- They have complete digestive, excretory, and circulatory systems.
- Their respiratory systems use gills or internal airways.
- They range in size from 0.1 mm to 1 m in length.
- They are extremely diverse and play vital roles in all aquatic and terrestrial ecosystems, often as the primary consumers.
- Insects are the only invertebrates capable of flight.
- About 1 100 000 species have been described (greater than 75 classes)
Phylum (common name): Nematoda (roundworms)
Examples (Animals shown in the photos are bolded*): pinworms, dog heartworms*
Key features:
- They have unsegmented cylindrical bodies with complete digestive tracts.
- Many are important parasites of other animals.
- Some females can produce more than 100 000 eggs each day.
- They live in very large numbers in the soil and aquatic sediments.
- They range in size from 2 mm to 2 m in length.
- About 20 000 species have been described (2 classes).
Phylum (common name): Annelida (segmented worms)
Examples (Animals shown in the photos are bolded*): earthworm, feather-duster worms*
Key features:
- Their bodies and most of their internal organs are segmented.
- They have complete digestive systems.
- Gas is exchanged through the skin, gills, or other specialized body parts.
- Most have bristles on their outer surface to help with movement.
- Many are marine.
- They range in size from 0.5 mm to 3 m in length.
- About 14 000 species have been described (7 classes).
Phylum (common name): Mollusca (mollusks)
Examples (Animals shown in the photos are bolded*): snails*, clams, octopuses, squid
Key features:
- They have three main unsegmented body parts: a foot, a visceral mass, and a mantle that secretes a shell.
- Almost all have specialized file-like radula used for scraping and boring.
- In some species the shell is reduced or absent.
- They have complete digestive systems, circulatory systems, and gills.
- They range in size from less than 1 cm to 20 m in length.
- About 120 000 species have been described (8 classes).
Phylum (common name): Rotifera (rotifers)
Examples (Animals shown in the photos are bolded*): rotifers*
Key features:
- They are small aquatic animals (less than 2 mm long).
- They use cilia to direct food into their mouths.
- Most live in fresh water.
- They have no respiratory or circulatory system.
- They are very important consumers in many aquatic food webs.
- About 1800 species have been described (3 classes).
Phylum (common name): Platy- helminthes (flatworms)
Examples (Animals shown in the photos are bolded*): tapeworms*, liver flukes
Key features:
- They are flattened, unsegmented worms.
- Most have a digestive cavity with a single opening.
- They do not have coeloms.
- They range in size from 1 mm to 5 m in length.
- Many are important parasites of other animals.
- They have no circulatory or respiratory system.
- About 14 000 species have been described (4 classes).
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Body Plans and Life Cycles
The body plans and life cycles of protostomes are extraordinarily diverse. Figure 10 shows a representative body plan from the most biologically successful class of organisms—Phylum Arthropoda, Class Insecta.
Figure 10 There are more known species of insects than of any other class of organisms on Earth. Their success is due in large part to an efficient body plan that includes nervous, digestive, respiratory, excretory systems and reproductive systems; advanced sensory structures; and the ability to fly.
Images: The external anatomy of a grasshopper: listing its small legs in front, and large legs in back, mouth parts, compound eye, antenna, head, thorax, wings, the hearing organ on front side under wings, and abdomen;
Internal anatomy of a female grasshopper: Brain, mouth, dorsal blood vessel, heart, ovary, malpighian tubules, digestive system, and ventral nerve cord with ganglia.
Other characteristic body features of protostomes include the powerful claws of crabs and lobsters, the tentacles of octopuses and squid, the ornate shells of snails and clams, and the large, colourful wings of butterflies.
Many animals, including insects, have a life cycle that involves a number of distinct larval stages. In these life cycles, the larval stages are sometimes unrecognizable when compared to the adult. Figure 11 shows examples of four larvae.
Figure 11 The larval stages of an (a) octopus, (b) snail, (c) butterfly, and (d) mosquito.
Human-Protostome Interactions
The protostomes include many species that are of great consequence to humans and interact with us in direct ways. They affect our food supply, our health, and our economy.
COMPETITORS
Many protostomes compete with humans for food. Insects are some of the best known competitors. Virtually every food crop has a number of important insect pests. For example, important pests of Ontario corn crops include at least 13 different species of insect and one species of slug (Phylum Mollusca) (Figure 12).
Figure 12 Tomato hornworms are common pests in Ontario. These caterpillars feed on tomato plants, including the fruit.
Food losses due to such competitors are significant, and the cost of controlling these pests can be a major factor in the cost of food production. Most crops are planted in large monocultures. This, along with the ability of many insects to reproduce quickly, allows insect populations to grow very quickly. Pesticides can keep pest populations in check, but some insects are becoming resistant to pesticides. Pesticides can also damage the environment and pose risks to human health. Organic and alternative farming methods can eliminate or reduce the use of synthetic pesticides. They rely instead on the presence of natural or introduced predatory insects to control pests.
Insects are also serious pests of other commercially valuable plants including trees and cotton. The spruce budworm, gypsy moth, Asian long-horned beetle, and mountain pine beetle are examples of insects that have caused significant damage to forests.
PATHOGENS AND VECTORS
Humans suffer from many parasitic diseases. Some are caused by protists, but others are caused by animals—particularly species of nematodes, tapeworms, and flukes. Various nematode parasites are capable of infecting many body tissues. Adult tapeworms inhabit intestines and pass their fertilized eggs out with feces (Figure 13). Parasitic flukes often live in lungs or blood vessels. One blood fluke that normally infects waterfowl is responsible for "swimmer's itch."
In addition to causing disease, many biting animals are vectors—they spread disease. For example, mosquitoes spread malaria, yellow fever, and West Nile virus from person to person. Ticks and fleas spread Lyme disease and the plague. Controlling the spread of these diseases involves controlling the animal vectors.
Figure 13 (a) A tapeworm has a specialized head, or scolex. (b) Some tapeworms have heads with barbs and suckers for attaching to the wall of the intestine.
FOOD AND ECONOMIC BENEFITS
Many protostomes benefit humans. We eat many of these animals, including mollusks such as clams and scallops, crustaceans such as shrimp and lobsters, and the honey produced by bees. We use oysters for food and as a source of pearls (Figure 14). Coral reefs are used in some countries as a source of minerals for cement production, and silk is a very valuable fibre produced by caterpillars of the silkworm moth. Perhaps the greatest direct value we receive from protostomes is the pollination of food crops by insects. If plants are not pollinated, they do not produce seeds, and many do not produce fruit.
Figure 14 Some oysters secrete a protective hard coating around a foreign particle, such as a grain of sand, forming a pearl.
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The Deuterostomes
The deuterostomes are a much less diverse but much more familiar collection of animals. This branch of the animal phylogenetic tree includes two main groups: the echinoderms (starfish, sea urchins, sea cucumbers, and their closest relatives) and the chordates (vertebrates and their closest relatives).
The Echinoderms
The 6500 species of echinoderms are the only invertebrates that exhibit the same pattern of early embryonic development as the vertebrates. The immature stages of echinoderms are free swimming and bilaterally symmetrical. As they grow, they develop a radially symmetrical pattern around five or more arms (Figure 15(a)).
Adult echinoderms have a unique body plan. They have a complete digestive system, a simple circulatory system, and no respiratory or excretory system. They do not have a head region, and their nervous system circles the mouth and extends into the arms. Echinoderms move using a water-filled vascular system that uses the principles of hydraulics. By pumping water with muscle action and opening and closing valves within the system, echinoderms can control the movements of their arms and tube feet (Figure 15(b)). This system also allows them to easily generate considerable force. Some starfish use this ability to pull apart large clams and feed on them.
Figure 15 (a) Echinoderms such as this brittle star are bilaterally symmetrical as larvae but radially symmetrical as adults. (b) Echinoderms move using a system of water-filled canals that control their arms and tube feet.
The Chordates
The chordates include fish, amphibians, mammals, reptiles, and birds. Although the birds are discussed here as a separate class, they are now considered a recently evolved group of reptiles.
The chordates include the most complex living organisms. They are thought to have evolved from bilaterally symmetrical ancestors with segmented bodies. These primitive chordates had gill slits, a dorsal nerve cord, and a flexible rod called a notochord. The notochord acted as an anchor for attaching the body wall muscles (Figure 16). These features probably enhanced the chordate's ability to move. In primitive chordates and many modern chordates the gill slits are used for filter feeding.
Figure 16 These are the distinguishing features of the common ancestor of all chordates. The simplest living chordates still retain this body form.
Image: The features of the chordates show the mouth at front, the dorsal hollow nerve cord inside top front, the notochord inside middle under dorsal hollow nerve cord, segmented body wall muscles surrounding inside parts, the anus exiting out back bottom, and the figestive tract which runs between the mouth and anus.
Over time a number of other important features evolved, including vertebrae, paired appendages, a bony skeleton, and more highly developed brain and sensory systems. Internal supporting skeletons enabled chordates to develop large body sizes—most vertebrates are many times larger than the vast majority of invertebrates. The blue whale, a vertebrate, is believed to be the largest animal that ever lived.
A key development that enabled some of the vertebrates to conquer land was the evolution of a waterproof amniotic egg. An amniotic egg has specialized membranes and an outer shell that make it resistant to water loss. Mammals, including humans, retain amniotic membranes around the developing fetus. Only a few species of mammals, including the platypus and the echidna, still lay shelled eggs.
amniotic egg: an egg with an outer leathery or hard shell and specialized internal membranes that protect and nourish the embryo
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The phylogenetic tree in Figure 17 indicates major features that arose in the evolutionary history of the chordates. The birds are shown as a distinct group, although they are now considered to belong to the reptile Glade.
Figure 17 The phylogenetic tree for the deuterostomes
- Echinoderms
- (notochord, segmented body, gill slits) Agnatha
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) Chondrichthyes
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) (bony skeleton) Actinopterygii
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) (bony skeleton) (two pairs of limbs) Amphibia
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) (bony skeleton) (two pairs of limbs) (amniotic egg, waterproof skin) Reptilia
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) (bony skeleton) (two pairs of limbs) (amniotic egg, waterproof skin) (feathers) Aves
- (notochord, segmented body, gill slits) (true vertebrae, jaws, paired appendages) (bony skeleton) (two pairs of limbs) (amniotic egg, waterproof skin) (milk, hair) Mammalia
THE VERTEBRATE SUCCESS STORY
Most chordates are vertebrates—their dorsal nerve cord is enclosed within a spinal column and cranium, or skull. The spinal column is divided into separate segments called vertebrae. Except for the earliest fish, all vertebrates have a well-defined head containing a cranium that surrounds and protects an enlarged brain. Unlike the external skeleton of arthropods, which the animal must shed in order to grow, the internal skeleton of vertebrates can grow along with the animal's body. The enlarged and protected brains and eyes, ears, and noses characteristic of vertebrates provide advanced sensory capability and enable coordinated behaviours and motion.
The majority of vertebrate species are fish. Their vertebrate skeletons and large brains enabled them to become powerful and agile swimmers, far surpassing the swimming capabilities of any invertebrates. Over time some fish evolved paired appendages, which gave rise to bony limbs. These fish were to be the ancestors of early amphibians and all land vertebrates.
Vertebrates evolved a number of key features that enabled them to conquer the land. Beginning with the limbs that allowed early amphibians to support their bodies on land, early reptiles evolved protective outer skin, enhanced lungs and circulatory systems, internal fertilization, and eggs with outer shells (Figure 18).
Figure 18 This leatherback sea turtle hatchling will scurry across the sand to reach the ocean, where it will spend most of its life.
Image: A leatherback sea turtle hatching from its egg.
Although greatly outnumbered by invertebrates, vertebrates are nonetheless a diverse and dominant group of animals. Vertebrates have the most advanced organ systems and the most complex behaviours of all organisms. These features, along with their large size, enable them to inhabit many aquatic and terrestrial environments. In addition, birds and mammals can maintain warm body temperatures. This lets them remain active in very cold environments when all other forms of life are dormant.
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Table 2 highlights the major taxonomic groups within the vertebrates. Notice that the first three groups are often collectively referred to as "fish," but they are actually quite distinct groups.
Table 2 The Vertebrates
Class: Agnathans (jawless fishes)
Examples (Animals shown in the photos are bolded*.): lamprey*, hagfish
Key features:
- Their skeletons are made of cartilage, they have no jaws, and a notochord is present in adults.
- They have gill slits but no paired appendages.
- Hagfish are scavengers.
- Many lampreys are parasites of fish.
- Lampreys have larval forms that are filter feeders.
- There are marine and freshwater species.
- About 60 species have been described.
Class: Chondrichthyes (cartilaginous fishes)
Examples (Animals shown in the photos are bolded*.): sharks*, rays, skates
Key features:
- Their skeletons are made of cartilage.
- They have jaws, vertebrae, gill slits, and paired appendages.
- Their fins are often thick.
- Many are predators of other fish.
- Reproduction uses internal fertilization.
- Most species are marine.
- About 850 species have been described.
Class: Actinopterygii (bony fishes)
Examples (Animals shown in the photos are bolded*.): most common fish—bass, trout*, tuna, salmon
Key features:
- Their skeletons are bony.
- Most have a swim bladder—a gas-filled organ used to control buoyancy.
- Their fins are often membrane-like with stiff "rays” for support.
- Most species use external fertilization.
- There are marine and freshwater species.
- About 24 000 species have been described.
Class: Amphibia (amphibians)
Examples (Animals shown in the photos are bolded*.): frogs, salamanders*, caecilians
Key features:
- Most have an aquatic larval stage with gills.
- Adults are tetrapods—having four limbs adapted for moving on land (except caecilians).
- Most species use external fertilization.
- They breathe through their lungs and/or skin.
- About 5000 species have been described.
Class: Reptilia (reptiles and birds*) *Birds are described separately.
Examples (Animals shown in the photos are bolded*.): snakes, lizards*, crocodilians
Key features:
- Most are terrestrial tetrapods with dry scaly skin.
- They breathe with lungs.
- They use internal fertilization.
- They have amniotic eggs with soft shells.
- Turtles lack teeth and have a bony shell. Aquatic species have webbed feet.
- Turtles are the most distantly related group within the reptile clade.
- About 7300 species have been described.
Class: Ayes (birds)
Examples (Animals shown in the photos are bolded*.): birds
Key features:
- They are tetrapods with forelimbs modified as wings.
- Most species are capable of flight.
- They have feathers.
- They are warm-blooded (endothermic).
- They have large brains and acute vision.
- They use internal fertilization.
- They lay hard-shelled amniotic eggs.
- About 9700 species have been described.
Class: Mammalia (mammals)
Examples (Animals shown in the photos are bolded*.): mammals
Key features:
- They are tetrapods and have hair.
- They nurse their young with milk produced in mammary glands.
- They are warm-blooded (endothermic).
- They have large brains and acute vision and sense of smell.
- They use internal fertilization.
- Most give birth to live young; a few species produce amniotic eggs.
- About 4500 species have been described.
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VERTEBRATE DIVERSITY AND THE WEB OF LIFE
Although greatly outnumbered by invertebrates, vertebrates are a diverse and important group of animals. When you hear the word "fish," "snake," "frog," "bird," or "mammal" you may envision "typical" examples. You might even think that the members of each of these groups are quite similar. How different is one kind of fish from another kind of fish, or one kind of snake from another? Is there really that much difference between frog or bird species? Do all mammals not have very similar characteristics?
As you have learned throughout this unit, species are grouped according to many shared features, but each species possesses a unique set of features that sets it apart from every other species on Earth. The examples listed in Table 3 are just a tiny sampling of the countless unusual and extraordinarily diverse vertebrates.
Table 3 Highlighting Vertebrate Diversity
Feature: Size
Examples:
- Blue whales can have a mass of more than 150 000 kg. Their hearts alone can have a mass of 600 kg. They give birth to a single calf with a mass of more than 2500 kg. The calf drinks 400 L of milk each day!
- The smallest bird is the bee hummingbird of Cuba. It has a mass of just 1.8 g, the same as a dime. This extraordinarily small flying machine is capable of beating its wings 80 times per second and visiting more than 1000 flowers in a single day.
Feature: Sensory ability
Examples:
- Echolocating bats can detect an echo off an object as small as a mosquito. They can also produce and detect sounds with frequencies far beyond the range of human hearing.
- Pit viper snakes have the ability to detect infrared radiation. Special pits on the sides of their heads detect this thermal energy, which is given off by warm-blooded mammals and birds.
Feature: Behaviour
Examples:
- The longest migration of any animal is that of the arctic tern. Their annual zigzagging return trip can cover a distance of over 71 000 km!
- Arctic ground squirrels, in contrast, hibernate over the winter. Arctic winters are so long that these ground squirrels spend more than 80 % of their lives asleep!
Feature: Life cycles
Examples:
- Finding a mate in the deep ocean is extremely difficult. So when the tiny male angler fish finds one, he bites onto the body of the female and remains attached to her for the rest of his life. The body of the female grows into the mouth of the male, and their circulatory systems fuse. When the female is releasing her eggs, hormones in her blood travel into the body of the male and cause it to release sperm.
- Females of the recently extinct gastric brooding frog swallowed their fertilized eggs. The eggs hatched in their mother's stomach, and the tadpoles developed there fully before climbing up her throat and hopping away!
Humans have a special interest in mammals and birds. This is partly because of their fascinating diversity of forms and behaviours, but it is no doubt also because we are vertebrates. This may explain why humans are much more likely to be concerned about threats to these species than they are about threats to invertebrate species or species in other kingdoms. Many people join organizations to help protect endangered mammals and birds, but few organizations focus on saving invertebrates, fungi, or plants.
Fortunately, most current efforts to protect the environment and the diversity of life on Earth focus on protecting habitats and entire ecosystems. Concern over species such as panda bears, polar bears, and gorillas captures public interest and media attention (Figure 19). If this interest is used to protect the ecosystems to which these wonderful species belong, the entire web of life within those ecosystems benefits. In this way, our interest in the vertebrates may be our most powerful means of protecting the diversity of life.
Figure 19 Roots and Shoots and the Jane Goodall Foundation promote environmental awareness and the protection of gorillas. Jane Goodall began her monumental work with wild gorillas in 1960.
Image: A mother gorilla sitting in the grass holding her sleeping baby gorilla in her arms.
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To learn about environmental organizations and how you can get involved, GO TO NELSON SCIENCE
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3.3 Summary
- Animals are an extremely large and diverse group of heterotrophic multicellular eukaryotes.
- Almost all animals are motile and have nerves that coordinate body movements and process sensory information.
- Animals are very diverse in their morphology and life cycles.
- Most animals develop from an embryo with three germ layers: the endoderm, the mesoderm, and the ectoderm.
- The simplest animals are the Porifera and the Cnidaria.
- Most animals are protostomes: arthropods, nematodes, mollusks, annelids, rotifers, or platyhelminthes.
- Humans have many important interactions with protostomes.
- The chordates are the most complex of all organisms and include the vertebrates: fish, amphibians, mammals, reptiles, and birds.
- The evolution of a bony skeleton, paired limbs, waterproof skin, and an amniotic egg enabled vertebrates to survive on land.
- The diversity of animals is threatened by many human activities, including habitat destruction, pollution, and climate change, but our affinity for animals motivates us to protect them and their habitats.
3.3 Questions
Key
K/U: Knowledge and Understanding
T/I: Thinking and Investigation
C: Communication
A: Application
1. Animals, plants, and fungi are all multicellular eukaryotes. What unifying characteristics distinguish animals from members of these other two kingdoms? K/U
4. Match the following traits to their animal group below. The traits are present in more than one group. K/U C
bilateral symmetry
nerves present
protostomes
asymmetrical
nerves absent
deuterostomes
annelids: (Blank Space)
chordates: (Blank Space)
rotifers: (Blank Space)
arthropods: (Blank Space)
porifera: (Blank Space)
nematodes: (Blank Space)
5. Which body system does each of the three germ ayers give rise to? K/U
7. State two key characteristics and two examples for each of the following phyla: K/U C
(a) Mollusca
(b) Nematoda
(c) Arthropoda
(d) Annelida
8. The platyhelminthes are considered simple but important animals. Justify this description. K/U C
11. What five major evolutionary advances are shared by all mammals, reptiles, and birds? Explain the benefit of each advance. K/U C
12. What special feature of birds and mammals has enhanced their ability to survive in cold climates? K/U
13. Marine ecosystems are vulnerable to human actions but often remain hidden from our view. The coral reefs in the Caribbean are subject to many human impacts and are in a general state of decline. Use the Internet and other sources to learn about the status of the Caribbean reefs. [GO TO NELSON SCIENCE] T/I
(a) What are the major human factors that have caused their decline?
(b) What is a lionfish? How is it influencing these reefs?
(c) How is climate change expected to affect these reefs?