21
Animal Kingdom: Animalia

Unicellular organisms are composed of only one cell. So, by definition, they lack tissues and organs, which are aggregations of differentiated cells. Nevertheless, unicellular organisms are extremely complex and often are classified as members of the animal kingdom. However, for reasons stated in the previous chapter, these forms are often lumped together and called protists.

Organisms belonging to the animal kingdom share a few fundamental characteristics. When fertilized, or stimulated to divide, the animal egg undergoes several divisions, or cleavages, producing a grape-like cluster of cells, the morula. As cleavage continues, the increased number of cells becomes arranged in the early embryonic stage known as a blastula, a single spherical layer of cells that encircles a hollow, central cavity. Following the formation of the blastula, the cells in this single-layered sphere undergo a series of complex movements establishing the shape and pattern of the early embryo. This transformation process is known as morphogenesis. In the process, the blastula becomes a simple double-layered embryo, the gastrula, from which the organism develops. The adult stages of the animal kingdom are multicellular. As adults, they are composed of tissues, albeit poorly defined tissues in the case of the sponges (Porifera).

All the animal phyla are thought to have evolved from a common ancestor. To date, about one million animal species have been described. Of these, the majority by far belong to the phylum Arthropoda, most of which are insects (class: Insecta). This chapter discusses all of the major phyla and some of the better-known lesser phyla.

LOWER INVERTEBRATES

Porifera

The phyllum Porifera, or sponges, are a group of about 10,000 species, which are aquatic, mostly marine, multicellular filter feeders that appear to have evolved from protozoans, independently from all the other multicellular phyla. The zygote develops into a multicellular, free-swimming, ciliated larva, which metamorphoses into an adult sponge by turning inside out, bringing its cilia inside. Often called poriferans, these relatively simple organisms have no organs, and their tissues are not well defined. Structurally, they consist of an outer layer of flattened epidermal cells. Inside this is a layer of wandering amoeboid cells and an innermost layer of collar cells, sometimes called choanocytes. The collar cells are flagellated. They produce water currents that flow from the external aquatic environment, through small pores in the sponge to the central cavity, the spongocoel, and out through a larger opening called the osculum. The microscopic food particles brought in with the currents of water are engulfed by the collar cells, which either digest these particles on their own or pass them to the amoeboid cells for digestion. The water currents also deliver oxygen to the cells and carry off carbon dioxide and nitrogenous wastes.

Small needle-like crystals, composed either of calcium carbonate or siliceous material (containing silica), are scattered throughout the body of the sponge. The crystals, which are called spicules, together with proteinaceous fibers, create the skeleton that helps these animals maintain their shape.

Sponges' usual modes of reproduction include asexual budding as well as sexual reproduction; both monoecious and dioecious forms occur. Although the larvae are ciliated and free-swimming, the adults are always sessile (sedentary) and are usually attached to a submerged object.

Cnidaria (Coelenterates): Hydra, Jellyfish, Corals, Sea Anemones

The phylum Cnidaria, sometimes called Coelenterata, includes about 9,000 species of jellyfish, hydras, sea anemones, sea fans, and corals. These organisms occur in several basic body forms. Jellyfish are free-swimming adult forms known as medusae, which usually look like rounded domes with tentacles hanging below. The polyp cnidarian body plan is basically an upside-down medusa. Unlike the motile medusae, polyps have the dome side on the bottom. They are attached to the substrate and therefore are sessile, with their tentacles pointing up, buoyed by the water. Hydra, coral polyps, and sea anemones are examples of the sessile adult cnidarians. Most cnidarians are marine, though there are many freshwater forms.

Radial symmetry, characteristic of starfish and jellyfish, means that on a line dividing these animals into two equal parts, they would be mirror images of each other. The entire phylum Cnidaria is composed of simple, radially symmetrical animals composed of an outer tissue layer, the ectoderm, and an inner endoderm. Between these is a gelatinous filling containing amoeboid and fibrous cells scattered throughout. This less distinct third tissue layer, the mesoderm, in Cnidaria is called the mesoglea.

The three tissue layers enclose a hollow interior, which is filled with water that enters and exits through one opening to the outside. By means of simple muscular movement, the jellyfish uses its body as a pump to propel itself through the water.

Cnidarian tentacles, attached around the “mouth,” have unique structures known as cnidoblasts, which are composed of nematocysts. These contain specialized harpoon-like structures connected to long threads that are discharged in response to chemical or tactile stimuli. Some of the nematocysts are sticky, barbed, or poisonous. They have the capacity, in the aggregate, to harpoon, lasso, or paralyze prey. Then the tentacles pull the food to the mouth.

With simple muscle contractions of the hanging tentacles, food is pulled up to the single opening and then into the interior, sometimes called the gastrovascular cavity, or coelenteron. There it is broken down with enzymes, enabling nutrients to be absorbed by the cells lining the cavity. These then pass the nutrients on to other cells. Waste products pass out the same opening through which the food enters. Beyond the gastrovascular cavity, there are no digestive organs. The nervous system consists of a nerve net with no centralization, meaning no central nervous system, and no head.

The only sense organs that any of these animals have are statocysts, receptor organs that inform the animal about gravity, and ocelli, light-sensitive organs that are groups of pigment cells and photoreceptor cells located at the base of the tentacles. These appear to constitute the first multicellular sense organs.

The hydras (class: Hydrozoa) are solitary and have only a polyp stage. However, many other hydrozoans are colonial and have a more complex life cycle, with a sedentary hydra-like stage, as well as a free-swimming jellyfish-like medusa stage. Many cnidarians have ciliated, free-swimming larvae known as planulae.

The true jellyfish (class: Scyphozoa) have a dominant medusa stage, although some also have a planula larva and a polyp stage in the life cycle.

The sea anemones, sea fans, and corals (class: Anthozoa) are all marine. None of the 6,200 members of this class has a medusa stage in the life cycle. They are more complex than the simple hydra-like polyps. The corals secrete a hard, limy skeleton that is a major component of all coral reefs.

Ctenophora: Comb Jellyfish

Members of the phylum Ctenophora, known as the ctenophores, or comb jellyfish, of which there are approximately 90 species, are very similar to the true jellyfish, being radially symmetrical animals with a sac-like body composed of an ectoderm, an endoderm, a mesoglea, and a gastrovascular cavity. Food is digested by digestive enzymes secreted from the cells lining the gastrovascular cavity. Indigestible material is voided through the mouth and through two small anal pores located near the single statocyst. What also sets ctenophores apart from the cnidarians are their mesodermal muscles. In addition, when tentacles are present, they have adhesive cells instead of nematocysts. Finally, they lack the polymorphic life cycle found among many of the cnidarians. Most ctenophores have eight rows of cilia; these are the combs running along the surface of their transparent body, which account for their ability to swim.

Platyhelminthes: Flatworms, Flukes, and Tapeworms

The 20,000 members of the phylum Platyhelminthes are the simplest animals to possess a bilaterally symmetrical body plan. This means that they have virtually identical right and left sides, with a different top and bottom (or front and back, depending on how the organism is oriented). They also have head and tail regions. “Headness” is known as cephalization, which is typical of most bilateral, active organisms. The head region contains sensory cells, such as nerve cells (neurons), and aggregations of nerve cell bodies (ganglia), which are considered an early step in the evolution of a brain.

Unlike the cnidarians, which have neurons dispersed in a loose network, termed a nerve net, many of the platyhelminthes have nerve cells arranged into long nerve cords that carry nerve impulses to and from the ganglia in the anterior end of the body. Many platyhelminthes have ocelli that differentiate not only light from dark but also the direction from which the light is coming. Many also form images. In addition, platyhelminthes possess chemoreceptors that enable them to locate food. Flatworms also have a tubular excretory system running the full length of the body. This system contains many small tubules that open at the body surface, where flame cells containing cilia help move water and waste materials out of the body.

Platyhelminthes have three tissue layers – the ectoderm, the mesoderm, and the endoderm – which constitute the cellular portion of the body. All organisms above the ctenophore level of organization have these three distinct tissue layers. Among most flatworms, these tissue layers enclose an internal digestive cavity that is lined by endoderm. Members of one small group of flatworms, however, known as the Acoela, lack a gastrovascular cavity.

In addition to possessing the tissue level of organization, flatworms also have the organ level of complexity. Platyhelminthes do not have a cavity, or coelom, between their digestive tract and body wall. Therefore, they, together with the nemertines are known as acoelomates. Nemertines (also called nemerteans because they are in the phylum: Nemertea) are commonly called ribbon worms and proboscis worms and are characterized by their proboscis, a long, muscular, tubular structure capable of being everted from the anterior region.

The platyhelminthes classes Turbellaria (free-living flatworms) and Trematoda (flukes) constitute parasitic flatworms that, unlike the planarians, lack cilia. In contrast to the turbellarians, which have an epidermis, flukes have a thick, highly resistant cuticle that protects them from the acids and enzymes of the digestive system(s) in which these parasites often live. Flukes, being internal and external parasites, attach to their animal hosts with characteristic hooks and suckers. In addition to their two-branched gastrovascular cavity, much of their volume consists of reproductive organs. Many flukes have complex life histories, often involving more than one host. Among some forms, the eggs pass through the intestinal cavity of their host and hatch in what is often an aquatic habitat.

The tapeworms, class Cestoda, are internal parasites, usually living in vertebrate intestines. They have a knob-like “head” structure called the scolex, which bears suckers, and many bear hooks as well, enabling them to become attached to their hosts. The long, ribbon-like body is usually divided into segments called proglottids, which are little more than reproductive sacs. Each proglottid contains both male and female sex organs. In time, they fill up with mature eggs. When ready, the segment detaches from the worm, moves through the host's intestines, and is passed with the feces. If an appropriate host eats food containing tapeworm eggs, the eggs hatch inside the host. The embryos bore through the intestines, enter the blood, and are carried to the muscles where they encyst, becoming encased in a hard protective coating. They remain in the muscles until another animal eats this intermediate host. Inside the new host, the cyst's walls dissolve and the young tapeworms develop, attach to the intestinal lining, and continue their life cycle. Intermediate hosts are involved in the life cycles of many tapeworm species.

Cnidarians, ctenophores, and platyhelminthes have a gastrovascular cavity with an opening that functions both as a mouth and as an anus. By observing the early embryonic state, called the gastrula, it has been determined that the first opening becomes the common mouth/anus opening in platyhelminthes. This first opening to form in the embryo is initially called the blastopore. This opening connects the internal cavity, known as the archenteron, with the exterior of the gastrula. However, in nemertines, while the blastopore becomes the mouth, an entirely different opening appears during embryonic development that becomes the anus. Similarly, it has been found that many other phyla, including the nematode worms, mollusks, annelids, arthropods, and velvet worms, share a similar type of development in which the blastopore becomes the mouth and another opening becomes the anus. For this reason, these organisms that develop in a distinctly different manner are often placed together in a group called the protostomes, or Protostomia (Ecdysozoa = a superphylum).

Other evolutionary lines that develop separate holes from the mouth and anus, where the blastopore becomes the anus and a separate opening becomes the mouth, include such phyla as the echinoderms, hemichordates, and chordates (which include the vertebrata or vertebrates). (Humans have a backbone or vertebral column, and therefore, we are vertebrates.) These phyla are collectively termed the Deuterostomia (deuterostomes).

In addition to the formation of the mouth and anus, there are many other differences between the protostomes and deuterostomes:

Cells in early cleavage stages in the protostomes are determinate; that is, the developmental fates of these first few embryonic cells are at least partially determined. The early cleavage stages in the deuterostomes are indeterminate. Because the developmental fates of the first few cells have yet to be determined, each cell can develop into an individual if separated at this early stage.
They have different cleavage patterns. That is, they differ in the ways in which the zygote divides into separate cells.
Larval types differ.
The mesoderm is formed from the ectoderm in what are sometimes referred to as the radiate phyla (the cnidarians and ctenophores). Beyond the radiate phyla are the more advanced protostomes and deuterostomes. In the protostomes, only a small amount of the mesoderm is formed from the ectoderm; most of it arises from cells located near the blastopore, between the endoderm and the ectoderm. In the deuterostomes, all of the mesoderm is of endodermal origin.
If a coelom is present, its formation differs with regard to the protostomes and deuterostomes. A true coelom is a cavity located between the digestive tract and the body wall, entirely surrounded by mesoderm. Coelomate protostomes have a coelom that forms from a split in the mesoderm. In coelomate deuterostomes the coelom forms from mesodermal sacs. These evaginate (grow out) from the wall of the archenteron, the cavity inside the gastrula stage of the early embryo, and become the digestive tract.

Aschelminthes: Nematodes (Roundworms), Rotifers

The aschelminthes, or sac worms, include small, often worm-like animals with a direct, rather straight digestive tract and a protective cuticle. None has a “head,” a respiratory system, or a circulatory system. Similar to the platyhelminthes, many have a flame cell excretory system, although the nematodes have a unique excretory system.

Nematoda includes all the nematodes or roundworms, of which there are over 80,000 known species, ranging in size from microscopic to over 3 feet in length. They are elongate roundworms that taper to a point at both ends. Unlike the flatworms, they don't have ciliary movement; instead, they thrash about by alternately contracting their longitudinal muscles. Since the nematode body cavity is not entirely enclosed by mesoderm, in this case represented by bands of muscle, they do not have a true coelom, which is why they are called pseudocoelomates.

Rotifera includes all the rotifers, which represent about 2,000 species of some of the smallest and most common aquatic organisms. They are named for the circle of cilia at their anterior end. The cilia beat in a manner that appears to be circular, like a rapidly rotating wheel. With the beating cilia, rotifers draw a current of water into their mouths, enabling them to capture unicellular organisms, which are then ground up by hard jaw-like structures. A posterior “foot” is used for attachment. Like the nematodes, rotifers are considered pseudocoelomate protostomes.

Lophophorate Phyla

The lophophorates are members of a few phyla of small protostomes, all of which have a lophophore, a specialized U- or horseshoe-shaped fold around the mouth with many ciliated tentacles attached. This structure creates a current of water that sweeps unicellular organisms and other tiny particles into a groove that leads to the mouth.

The digestive tracts in many of these organisms are U-shaped, with the anus lying outside the crown of tentacles. All the organisms included in these phyla (Phoronida, Ectoprocta, Bryozoa, and Brachiopoda) are aquatic and most are marine. Their larvae are ciliated and free-swimming. Eventually they settle down to the substrate, where they secrete a protective case and remain sessile the rest of their lives.

The phoronids amount to about 15 worm-like organisms. There are about 5,000 species of ectoprocts; most are small, colonial, sessile organisms. There are over 5,000 species of bryozoans, and the brachiopods represent a group of 335 hard-shelled animals that look something like bivalved mollusks (clams). However, the relationships between brachiopods and other invertebrate phyla remain uncertain. Brachiopods were far more common millions of years ago; in fact, over 30,000 extinct species have been described.

HIGHER INVERTEBRATES

Mollusca: Snails, Clams, Octopuses, Chitons

Mollusca and the next phylum, Annelida, are both protostomous. It is thought that they both evolved from a segmented common ancestor, which was divided into repeated sections by a series of partitions called septa. However, unlike the annelids, the mollusks lost their segmentation. Mollusks and annelids possess a coelom and a circulatory system. The coelom divides the muscles of the gut from those of the body wall, enabling both sets of muscles to move independently. Development of the coelom was paralleled by the development of a complex circulatory system, made possible by the coelomic space where fluids bathe the organs without being squeezed out by the surrounding muscles.

The mollusks are one of the more successful animal phyla in terms of species numbers, totaling about 110,000 in all. Most forms have what is termed a foot, the muscular organ upon which they move. Between the foot and the mantle, which is the outermost layer of the body wall, are the internal organs. The mantle of most mollusks secretes a calcium-containing shell, although some forms such as slugs and octopuses have lost their shells. Others have only reduced, modified versions of what were once more pronounced versions of a shell.

Mollusks share many of the same features. They are bilaterally symmetrical and most have gills. Some have an open circulatory system, while others have one that is closed. Mollusks have considerable cephalization with both central and peripheral nervous systems. They are dioecious, and their organ systems include specialized structures that possess nephridia. Many mollusks have a larval stage that is quite similar to that of marine turbellarians.

Despite the many shared molluscan features, this phylum is also extremely diverse. Although there are seven major groups (classes), the best known are those in Gastropoda, which includes the snails, slugs, nudibranchs, conchs, abalones, and whelks. These organisms have either one shell or have lost it entirely. The class Bivalvia includes what are more commonly known as the bivalves, those “shells” with two shells or valves. Among these are the clams, oysters, scallops, and mussels. The Cephalopoda include the octopuses, squid, cuttlefish, and nautiluses. The chitons are members of the class Amphineura. Generally regarded as the most primitive of the living mollusks, their body plan is the closest to what the first mollusks are thought to have looked like. Their peculiar, segmented shell distinguishes them from all other molluscan classes; it consists of eight serially arranged dorsal plates.

Annelids

Annelid worms, which are also called ringed worms or segmented worms, have ring-like external segments coinciding with internal partitions. The approximately 22,000 species of these worms have been classified into three major groups. The earthworms, also called terrestrial bristle worms (class: Oligochaeta), are mainly terrestrial and freshwater scavengers that burrow in moist soil. The marine bristle worms (class: Polychaeta) are marine annelids that typically possess a distinct head with eyes and antennae. Usually, each of the serially arranged body segments has a pair of lateral appendages called parapodia for both locomotion and gas exchange. Most polychaetes move about by either swimming or crawling and are often found under things or in the mud and sand. Others are sedentary, living in tubes. Some of the tube dwellers have a crown of colorful, feather-like processes.

Believed to have evolved from oligochaetes, the class Hirudinea contains the group commonly called leeches. Though sometimes called bloodsuckers, many are not parasitic. They are flattened and possess a sucker at each of the tapered ends. The blood-sucking forms attach themselves with the posterior sucker; with the anterior sucker they either pierce through the host's skin with their sharp jaws or they dissolve the host's flesh with enzymes. Blocking the clotting process by secreting the anticoagulant hirudin, they continue to ingest the steady flow of blood.

Most annelids have digestive tracts that are straight, running from the mouth along the entire length of the body to the anus. Some annelids have gills, though these are usually the marine forms with parapodia. Inside the parapodia are capillary beds that function in gas exchange. The blood transport system is closed with hearts located only along the main vessels. Blood is then circulated through many smaller, adjacent vessels. Water and salt regulation is maintained by many kidney-like structures called nephridia. Their reproductive systems are well developed: Some species are dioecious, others monoecious, and some earthworms are known for their hermaphroditism, meaning that each individual has both male and female reproductive parts. Less well known is the fact that they still need to mate with one another because their organs are aligned in such a way that they are unable to mate with themselves. Therefore, these earthworms line up together with their reproductive organs meeting in what amounts to a simultaneous double mating.

Asexual reproduction is common among many forms of annelids as well. This is usually accomplished when a parent worm breaks into two or more segments that regenerate. The process is called fragmentation. Often, even before the adult divides, regeneration precedes the separation, so that a new zone of cells begins to form head and tail parts in the appropriate position. Such zones of regeneration are called fission zones.

Onychophora

The small phylum Onychophora, which are also called velvet worms, and peripatus, includes only about 80 species worldwide, and all are restricted to the tropics. Their long worm-like bodies are similar to caterpillars because they move on many pairs of short, unjointed legs. Onychophorans have a thin, flexible cuticle, much like that of the annelids, rather than a harder, jointed, less permeable, more arthropod-like cuticle. Like the annelids, onychophorans have a pair of nephridia in each body segment, but like the arthropods, they have an open circulatory system. Modern onychophorans also possess a tracheal respiratory system that is something like that found among many of the terrestrial arthropods; however, it is thought that these systems evolved independently. Unlike caterpillars, onychophorans do not metamorphose into winged moths or butterflies. Instead, onychophorans look like caterpillars throughout their adult lives.

Arthropoda: Crustaceans, Spiders, Mites, Ticks, and Insects

The phylum Arthropoda includes more species than any other phylum, with over 900,000 described insect species. Some researchers feel that this may represent only a fraction of their total number, claiming that there might be as many as 10 million living species of insects. It is thought that most species inhabit the tropical rain forests. Insects account for the majority of all the arthropodan species.

In much the same way that the chordates (which include the vertebrates) represent the most successful group of deuterostomes, arthropods are the most diverse and successful group of protostomes.

The tough external cuticle covering arthropodan bodies functions as an external skeleton, or exoskeleton. The cuticle is composed of proteins and the strong but flexible polysaccharide chitin. Attached to the inside of the many jointed, hinged parts, or plates, are muscles that enable the organism to move while being covered with a protective armor. Equally as important is the exoskeleton's value as waterproofing. The exoskeleton has enabled arthropods to become one of the most successful groups to colonize terrestrial habitats.

Although the exoskeleton has helped make arthropods the most successful animals ever (in terms of total number of species), the main drawback to the arthropodan exoskeleton is that it restricts growth, weight, and size. To grow beyond a certain point, an organism must periodically detach the muscles connected to the interior of the exoskeleton, shed its cuticular layer, expand in size, and then lay down a new hard outer covering to which the muscles are reattached. Growth, therefore, is made possible through a series of molts during an arthropod's lifetime. These successive molts also give the organism an opportunity for morphological change. This is especially true of the young. Larvae of many arthropods go through a succession of genetically controlled changes, thereby incrementally becoming more adult-like. Other species undergo a more rapid transformation, or metamorphosis, in just one molt. This is the case with caterpillars, which pass through their pupal stage before becoming a butterfly.

The generalized arthropodan body plan is slightly elongate with a segmented body somewhat reminiscent of its annelid-like ancestry. During the course of evolution, many arthropods lost much of their segmentation. In some forms, the segments became grouped together to form distinct body sections.

Picture of an American cockroach that has 3 major body segments, the head, thorax, and abdomen. The head has a pair of antennae, mouthparts, and compound eyes. The thorax has three pairs of legs and, one or two pairs of wings.

The open arthropodan circulatory system, with an internal cavity, the hemocoel, bathed with its equivalent of blood, hemolymph, was described in Chapter 11, on internal transport systems.

One of the major arthropod classes is the Arachnida (spiders, scorpions, mites, and ticks). Arachnids have six pairs of jointed appendages, the most anterior of which, the chelicerae, are adapted for manipulating, piercing, and sucking out their prey's fluids. Sometimes poison glands are associated with this first pair of appendages. The next pair, the pedipalps, are sensory. They are sensitive to touch and are capable of detecting certain chemicals; they also help to manipulate food. The other four pairs of appendages are used for locomotion.

Within the phylum Arthropoda is the class Crustacea (lobsters, shrimp, crabs, crayfish, barnacles, wood lice, pill bugs, and water fleas). Most forms are aquatic, though some, such as the pill bugs, are terrestrial. They have two pairs of antennae, three pairs of feeding appendages adapted for chewing, and a number of pairs of legs. This is a diverse class in terms of the number and arrangement of their appendages. They are also extremely variable in terms of size: some are small planktonic forms; others are huge crabs.

The major class in this phylum is Insecta. Most insects are terrestrial, and those that are aquatic appear to have evolved from terrestrial forms. Their arthropodan segmentation is reduced to three major body segments, the head, thorax, and abdomen. The head has a pair of antennae, specialized mouthparts, and compound eyes. The thorax has three pairs of legs and, often, one or two pairs of wings. The abdomen contains the viscera and reproductive organs (see Figure 21.1).

Like other arthropods, insects have specialized excretory Malpighian tubules and secretory glands that maintain water and salt balance. These structures accomplish functions analogous to those carried out by vertebrate kidneys.

Insects, like other arthropods, are dioecious. Breathing is accomplished through a series of pores, the spiracles, which carry air through a series of smaller and smaller branching tubes, the trachea, to all the cells of the body. Other arthropod classes include Chilopoda (centipedes) and Diplopoda, which are commonly called the millipedes. Both of these classes have bodies that are divided into a head and trunk region. The latter part is elongate, sometimes somewhat flattened, and divided into many segments. One of the main differences between centipedes and millipedes is that the former have a single pair of legs on each trunk segment while the latter have two pairs of legs per segment, that is, two legs on each side of the body segment.

Echinodermata

There are about 7,000 species of echinoderms. This entire group represents one of the two major deuterostomous phyla. The radially symmetrical animals in the phylum Echinodermata include the crinoids, starfish, sea urchins, sea cucumbers, brittle stars, and sand dollars in this group of “spiny-skinned” organisms, the literal meaning of Echinodermata. Each has a series of calcareous spines and plates located just under the skin. They also have pentaradial symmetry, with adult bodies divided into five parts around a central disc. It is usually underneath this central disc that the mouth is located.

Also unique to the echinoderms is their water vascular system, a series of fluid-filled vessels that use hydraulic pressure to operate the hollow tube feet or podia, each of which has a suction tip that can be attached or removed from objects. When coordinated, the tube feet manage feeding and locomotory functions.

All echinoderms are marine, usually bottom-dwelling, and most are motile. The major classes include the Crinoidea (sea lilies and feather stars). These have branched, feathery arms that use mucus to trap floating food particles, which are then brought to the mouth by means of the ciliated tube feet.

The Asteroidea (sea stars) are what most people call starfish. They are flattened, with five or more arms radiating from the central disc. Most are carnivorous, often feeding on crustaceans and mollusks, which they can open up with sustained pressure applied with their tube feet.

The Ophiuroidea (brittle stars and serpent stars) include animals that look something like the sea stars except that they have longer, thinner, more flexible arms branching off from their central disc. The common name comes from their arms, which are very brittle, and snap off when handled. Ophiuroidea eat detritus, organic debris that settles on the bottom, which is then digested in their large, simple, sac-like stomach. They have no intestine or anus.

Echinoidea (sea urchins and sand dollars) represent roundish, sometimes almost spherical organisms (e.g. sea urchins), and sometimes flattened organisms (e.g. sand dollars) that live mouth-down on the bottom. Unlike the ophiuroids, they have an intestine and anus in addition to a mouth. Sea urchins are usually protected by spines ranging in size from those that are quite short to some forms with long, thin, pointed spines that can penetrate soft skin. The calcareous plates have holes through which the tube feet protrude.

Holothurioidea (sea cucumbers) look like cucumbers. They don't have any arms but are able to move slowly on their slightly flattened ventral side with their tube feet. Many forms eat sand, filtering out the organic particles and passing the rest through their anus.

Embryological studies have revealed that the echinoderms and the next group, the chordates, share many characteristics. Therefore, researchers place them on an evolutionary branch that diverged long ago from the protostomes.

Hemichordata: Acorn Worms

There is one more invertebrate phylum, Hemichordata, to describe before reaching the chordates. This represents about 85 marine animals that look something like fairly large worms, most of which live in U-shaped burrows. Their conical proboscises are acorn-like – hence the name acorn worms.

They are particularly significant because they share some characteristics of both the echinoderms and the chordates. Therefore, they are sometimes thought of as an offshoot from an early common ancestor. Along the wall of their pharynx is a series of gill slits, one of the key characteristics that identify the chordates. In addition, they have a ciliated larval stage that is very similar to that of some echinoderms.

CHORDATA

Urochordata: Tunicates and Sea Squirts and Cephalochordata: Lancelets

The phylum Chordata is usually divided into three subphyla, the Urochordata (tunicates), Cephalochordata (lancelets), and the Vertebrata (vertebrates). The first two subphyla are considered the invertebrate chordates, although each contains, during some stage in its life history, a rod-like notochord that acts as an internal skeleton. In the vertebrates, it is the notochord that is surrounded or replaced by vertebrae. All three subphyla have gill slits in their pharynx (pharyngeal gill slits) at some point during their lives. They also have a dorsal hollow nerve cord, unlike other animals with a main nerve cord that is ventrally located.

Urochordata (tunicates and sea squirts) have tadpole-like larvae with a notochord and a dorsal hollow nerve cord. During metamorphosis, the notochord is resorbed and the larva expands in girth into a sessile adult tunicate, which is little more than a sedentary, filter-feeding pharyngeal basket.

Only 29 species in the subphylum Cephalochordata are known. Although similar to the urochordate tadpole larvae just described, the lancelets (Amphioxus) have a much larger gill system containing many more gill slits. They feed like urochordates, using cilia to create a water current that carries particulate matter into their mouths. While the water passes out the gill slits, the food particles trapped in mucus pass down the pharynx into the alimentary canal, where they are digested.

Vertebrata

Vertebrates (Vertebrata) have a vertebral column that distinguishes them from other chordates. Their vertebrae form the backbone that is the supporting axis holding up the body and protecting the spinal cord. Together, the seven classes composing this group total about 43,000 species, of which nearly half are fish (superclass: Pisces). The fish comprise three classes, Agnatha (lampreys and hagfish), Chondrichthyes (sharks and rays), and Osteichthyes (bony fish). The other four classes are the amphibians (Amphibia), reptiles (Reptilia), birds (Aves), and mammals (Mammalia).

Pisces: Fish

Agnathans were originally filter feeders, straining mud and water through their mouth and out their gills. The only living members are highly modified, having lost their bone and replaced it with cartilage. Both lampreys and hagfish are jawless, with disc-shaped mouths that are either rasping or sucking. Their gills are internal to their gill arches. They lack scales and, unlike the other two classes of fish, they lack paired fins. Lamprey larvae are filter-feeding and remarkably similar to Amphioxus.

Chondrichthyes include the sharks, skates, rays, and chimeras. They all have cartilaginous skeletons, paired fins, and jaws derived from gill arches. Living forms have teeth and small scales.

All the other fish are members of the group Osteichthyes. Most are bony and all have jaws (see Figure 21.2). An early offshoot of this group eventually led to the first amphibians.

Amphibia: Amphibians

The earliest amphibians were quite fish-like. Although they had lungs and leg-like appendages, they probably spent most of their time in the water. Slowly they exploited nearby terrestrial habitats, but specific characteristics restricted their advancement, evolutionarily speaking.

Picture of a fish variety called striped bass that has internal gills, fins, and body scales. — **Figure 21.2** Fish: Striped bass.

They all laid fish-like eggs that were generally exposed and susceptible to rapid desiccation. Therefore, these eggs needed to be laid in a moist place and were usually deposited in the water. From behavioral studies of modern amphibians, it is thought that most early amphibians were external fertilizers; that is, sperm was deposited and fertilization occurred after the eggs had been laid. This had the effect of restricting the breeding of amphibians to times when the males and females could meet in the same spot where the eggs are laid. Most modern amphibians are external fertilizers, though some have evolved modes of internal fertilization.

During development, the norm is for the young to pass through a gilled larval stage, although in a very few cases, the larval stage has been circumvented through specialized developmental modifications. Most adult amphibians have a rather permeable skin that renders them susceptible to desiccation. Amphibians played a dominant role in life on earth during the Carboniferous period, often called the Age of Amphibians, 280–360 million years ago. However, they slowly declined as members of a new class of chordates, the reptiles, replaced them.

Of those amphibians to survive, the groups that are still represented include the salamanders (order: Apoda) and the frogs (order: Anura). Together these living amphibian species total about 2,500, and new species, especially frogs, continue to be discovered (see Figure 21.3).

Reptilia (Reptiles)

Evolving early on from primitive amphibians, the reptiles expanded in numbers and importance until the Mesozoic era, or the Age of Reptiles (or the Age of Dinosaurs), 252 to 66 million years ago, after which their dominance declined. The characteristics that enabled them to become so successful included both the shelled egg and the embryonic membrane, known as the amnion, which the amphibians lacked. The amnion is the innermost membrane in the reptilian egg. It is also found in bird eggs as well as among mammals. The embryo, located within the amnion, is bathed in amniotic fluid, which protects the developing animal from mechanical injury. It seems to have been an important development that, in addition to the outer shell, enabled reptiles to lay terrestrial eggs, freeing them from their amphibian and fish ancestry, which required aquatic egg-laying.

Picture of a green frog that belongs to the amphibian family that has lungs and leg-like appendages, and are found mostly in water. — **Figure 21.3** Amphibian: Green frog.

Reptiles are internally fertilized. The male inseminates the female with sperm. Then the female can go off and lay her eggs at another time. In some instances, the sperm can live over a year inside the female, giving her considerable freedom with regard to where and when she oviposits (lays her eggs). When the eggs hatch, unlike the amphibians, the young are basically small versions of the adults, never having to pass through a larval stage. Since amphibian larvae are tied to the water, this advancement created another factor that freed reptiles from their aquatic ancestry.

Some reptiles were largely terrestrial; their dry, scaly, relatively impermeable skin substantially reduced their susceptibility to water loss, allowing them to inhabit environments that were considerably more arid than those occupied by most other vertebrates.

Other differences include the reptilian legs, which are usually larger and stronger and are oriented less laterally than those of the amphibians, enabling many reptiles to carry their bodies off the ground. This lateral arrangement appears to have been better for more rapid, terrestrial locomotion. Reptilian lungs are better developed than those of amphibians, and greater rib musculature enables better lung ventilation. Their four-chambered heart is better equipped to cope with the demands of a terrestrial existence.

Picture of a garter snake of the reptile family that can live both on land and in water. — **Figure 21.4** Reptile: Garter snake.

The four surviving orders of modern reptiles are the turtles (order: Chelonia), crocodiles and alligators (order: Crocodilia), lizards and snakes (order: Squamata), and the tuatara, which is the sole surviving member of an ancient order, Rhynchocephalia. Together, there are about 6,500 species of reptiles (see Figures 21.4 and 21.5).

Picture of a snapping turtle of the reptile family that can live both on land and in water. — **Figure 21.5** Reptile: Snapping turtle.

Picture of a black-crowned night heron that belongs to the bird family; they regulate their body temperature, in large part by means of their metabolism. — **Figure 21.6** Bird: Black-crowned night heron.

Aves: Birds

There are between 9,000 and 10,000 species of living birds. The earliest bird fossils are those of Archaeopteryx, which appear in 150-million-year-old deposits. These animals had wings and feathers. But, unlike modern birds, Archaeopteryx had teeth and a long bony tail. Slightly older, and quite similar fossils of Aurornis xui, which were not birds, are 160 million years old. The Aurornis xui was a small, feathered dinosaur with a long tail, toothed jaws, and clawed hands. Modern birds differ from the above in that they lack teeth and their bones are hollow (which makes them lighter). Both of these are adaptations for flight. Like reptiles, birds have a four-chambered heart. All have internal fertilization and land-adapted eggs with calcium carbonate shells. They are homeothermic. That is, they regulate their body temperature, in large part by means of their metabolism (see Figure 21.6).

Mammalia: Mammals

Mammals also evolved from an early group of reptiles. The oldest mammalian fossils are 200 million years old, from the Triassic. Reptiles were still dominating the earth when the first mammals evolved, but it was during this time that mammals established many of the characteristics that eventually enabled them to do so well when the opportunities became available (see Figure 21.7).

It was at the end of the Cretaceous period, about 65 million years ago, that many of the dominant groups of reptiles became extinct over a span of several million years. This period of extinctions was accompanied by increased opportunities that led to an increase in mammalian numbers as well as species.

Picture of an eastern chipmunk which is a mammal with fur that functions as insulation to its body. — **Figure 21.7** Mammal: Eastern chipmunk.

Like the birds, mammals have a four-chambered heart. Both mammals and birds are homeothermic, or warm-blooded. Both bird feathers and mammal hair and fur function as insulation. Other characteristics that differentiate the mammals from the reptiles include their diaphragm, the muscle under the rib cage that significantly improves their breathing efficiency. Specific skeletal features are unique, as well as certain anatomical features, such as their greatly enlarged cerebrum. Mammals do not lay eggs, except for the monotremes, which include the platypus and the spiny anteater. After birth, mammals nourish their young with the milk that is secreted from the mother's mammary glands.

KEY TERMS

abalones	amphibians
abdomen	Amphineura
Aceola	Amphioxus
acoelomates	animal kingdom
acorn worms	annelid worms
adhesive cells	Annelida
Agnatha	Anthozoa
agnathans	Anura
alligators	Apoda
amnion	Arachnida
amoeboid cells	Archaeopteryx
archenteron	chitons
Arthropoda	choanocytes
aschelminthes	Chondrichthyes
asexual budding	Chordata
Asteroidea	cilia
Aves	ciliated larvae
barnacles	circulatory system
bilateral symmetry	clams
birds	cleavages
Bivalvia	closed circulatory system
blastopore	Cnidaria
blastube	cnidoblasts
blastula	Coelenterata
bony fish	coelenterates
Brachiopoda	coelenteron
brachiopods	coelom
brittle stars	collar cells
Bryozoa	colonial
calcareous spines and plates	comb jellyfish
calcium carbonate	combs
centipedes	conchs
central nervous system	coral polyps
cephalization	corals
Cephalochordata	crabs
Cephalopoda	crayfish
Cestoda	Crinoidea
Chelonia	crinoids
chemoreceptors	crocodiles
Chilopoda	Crocodilia
chimeras	Crustacea
chitin	crustaceans
Ctenphora	frogs
ctenophores	fur
cuticle	ganglia
cuttlefish	Gastropoda
Deuterostomia	gastrovascular cavity
diaphragm	gastrula
dioecious	gill slits
Diplopoda	gills
dorsal hollow nerve cord	hagfish
earthworms	hair
Echinodermata	head
echinoderms	Hemichordata
Echinoidea	hemocoel
ectoderm	hemolymph
Ectoprocta	hermaphroditism
ectoprocts	hirudin
encyst	Hirudinea
endoderm	Holothurioidea
epidermal cells	homeothermic
exoskeleton	hydra
feathers	Hydrozoa
fibrous cells	Insecta
fish	insects
fission zones	internal digestive cavity
flame cell excretory system	internally fertilized
flame cells	jellyfish
flatworms	lampreys
flukes	lancelets
foot	leeches
fragmentation	lizards
free-living flatworms	lobsters
lophophorates	notochord
Malpighian tubules	nudibranchs
mammals	ocelli
mammary glands	octopuses
mantle	Oligochaeta
marine bristle worms	Onychophora
medusae	onychophorans
mesoderm	open circulatory system
mesodermal muscles	Ophiuroidea
mesoglea	osculum
metamorphosis	Osteichthyes
millipedes	oysters
mites	parapodia
Mollusca	pentaradial symmetry
monoecious	peripheral nervous system
morphogenesis	permeable skin
morula	Phoronida
multicellular	phoronids
mussels	pill bugs
nautiluses	Pisces
nematocysts	planulae
Nematoda	Platyhelminthes
nematodes	Polychaeta
Nemertea	polyp
nemerteans	Porifera
nemertines	proboscis
nephridia	proboscis worms
nerve cords	proglottids
nerve net	proteinaceous fibers
neurons	Protostomia
pseudocoelomates	slugs
radial symmetry	snails
radiate phyla	snakes
rays	spicules
regeneration	spiders
reptiles	spiracles
Rhynchocephalia	sponges
ribbon worms	spongocoel
Rotifera	Squamata
rotifers	squid
roundworms	starfish
sac worms	statocysts
salamanders	tapeworms
sand dollars	tentacles
scallops	terrestrial bristle worms
scolex	thorax
scorpions	ticks
Scyphozoa	tissues
sea anemones	trachea
sea cucumbers	tracheal respiratory system
sea fans	Trematoda
sea squirts	tubular execretory system
sea urchins	tunicates
secretory glands	Turbellaria
septa	turtles
sessile adults	Urochordata
sexual reproduction	Vertebrata
sharks	vertebrates
shell	water fleas
shelled egg	water vascular system
shrimp	whelks
silica	wood lice

SELF-TEST

Multiple-Choice Questions

Sponges

Sponges are multicellular
filter feeders that have ______________ organs.
1. no
2. one
3. two
4. three
5. four
The poorly defined tissue layers of poriferans include ______________.
1. an outer layer of flattened epidermal cells
2. a layer of wandering amoeboid cells
3. an innermost layer of collar cells
4. all of the above
5. none of the above
Sponges have a central cavity called the ______________.
1. osculum
2. spicule
3. spongocoel
4. gastrovascular cavity
5. stomach
Water leaves the spongocoel via the ______________.
1. osculum
2. spicule
3. spongocoel
4. gastrovascular cavity
5. stomach
The water currents also deliver oxygen to the cells and carry off ______________ and ______________.
1. carbon dioxide, nitrogenous wastes
2. chlorophyll, carotenoids
3. leucocytes, lymphocytes
4. all of the above
5. none of the above
Small needle-like crystals of either calcium carbonate or siliceous material, known as ______________, are scattered throughout the body of sponges creating a skeleton of sorts that helps maintain their shape.
1. spongocoels
2. osculums
3. spicules
4. epidermal nodules
5. mesodermal nodules
Adult sponges are ______________.
1. mobile
2. ciliated
3. sessile
4. all of the above
5. none of the above

Cnidaria and Ctenophora

The free-swimming
form found among cnidarians that usually looks like a rounded dome with hanging tentacles is known as a ______________.
1. polyp
2. medusa
3. snail
4. mesoglea
5. planula
Cnidarians have a sessile stage that is attached to the substrate, with the tentacles pointing up, which is known as a ______________.
1. polyp
2. medusa
3. snail
4. mesoglea
5. planula
The outer layer of a cnidarian is composed of the tissue known as ______________.
1. mesoderm
2. phisoderm
3. endoderm
4. placoderm
5. ectoderm
Between the cnidarian's outer and inner layers is a gelatinous filling comprising a third layer, containing amoeboid and fibrous cells scattered throughout; this layer is called the ______________.
1. mesoglea
2. phisoderm
3. endoderm
4. placoderm
5. ectoderm
Before the food is absorbed by the cnidarians' cells, it is broken down in the hollow interior called the ______________.
1. gastrovascular cavity
2. coelenteron
3. spongocoel
4. all of the above
5. a and b
Unique to the cnidarians, their tentacles are armed with ______________ containing ______________, which are like harpoons connected to long threads that are discharged in response to chemical or tactile stimuli.
1. statocysts, ocelli
2. nematocysts, cnidoblasts
3. statocysts, nematocysts
4. nematocysts, ocelli
5. cnidoblasts, nematocysts
The only sense organs that any of the cnidarians have are ______________ and ______________.
1. statocysts, ocelli
2. cnidoblasts, nematocysts
3. statocysts, statoblasts
4. planularians, hydrolarians
5. scyphozoa, anthozoa
Many cnidarians
have ciliated, free-swimming larvae known as ______________.
1. medusas
2. polyps
3. planulae
4. tadpoles
5. hydras
The ______________ are very similar to the true jellyfish, being radially symmetrical animals with a sac-like body form composed of an outer ectoderm and an inner endoderm with a mesoglea and gastrovascular cavity. However, they don't have nematocysts, and most have eight rows of cilia (combs) running along the surface of their transparent body, enabling them to swim.
1. Scyphozoa
2. Anthozoa
3. Hydrozoa
4. Ctenophora
5. Cnidaria

Platyhelminthes, Nemertea

Flatworms have a tubular excretory system running the full length of the body. This system contains many small tubules that open at the body surface where ______________ that contain cilia help move water and waste materials out of the body.
1. chemoreceptors
2. nerve net
3. ganglia
4. kidneys
5. flame cells
Platyhelminthes, along with another similar phylum, the Nemertea, are termed ______________ because they do not have a cavity between their digestive tract and body wall.
1. coelomates
2. acoelomates
3. protostomes
4. deuterostomes
5. proboscis worms
Tapeworms have a knob-like “head” structure termed the ______________, bearing suckers and often hooks as well.
1. ganglion
2. proglottid
3. scolex
4. osculum
5. spicule
Tapeworms, members of the platyhelminthes class, Cestoda, have a very long ribbon-like body that is usually divided into segments called ______________.
1. ganglia
2. metamers
3. septa
4. scolexes
5. proglottids
Both the
cnidarians and the platyhelminthes have a gastrovascular cavity with one opening that functions as both a mouth and an anus. During embryonic development, it is the ______________ that becomes the mouth and anus in these organisms.
1. gastropore
2. archenteron
3. protostomia
4. deuterostomia
5. blastopore
It has been shown that in nemertines (proboscis worms), the blastopore becomes the mouth and an entirely different opening, which appears during embryonic development, becomes the anus. This evolutionary line, where the blastopore becomes the mouth and another opening becomes the anus, is often called the ______________.
1. lower invertebrates
2. higher invertebrates
3. rhynchocephalia
4. protostomia
5. deuterostomia
Another evolutionary line in which the blastopore becomes the anus and a new mouth is formed is termed the ______________.
1. cleavagestomia
2. blastopores
3. protostomia
4. deuterostomia
5. cnidaria

Aschelminthes, Lophophores

Nematodes are long roundworms that taper to a point at both ends. Unlike the flatworms, they don't have ciliary movement, but, instead, they thrash about with their longitudinal muscles. Since their body cavity is not entirely enclosed by mesoderm, they do not have a true coelom, and are termed ______________.
1. acoelomates
2. coelomates
3. pseudocoelomates
4. gastrocoels
5. enterocoels
There are a few phyla of small protostomes, all having a ______________, which is a specialized fold around the mouth, usually U-shaped, that has many ciliated tentacles attached. These groups are often called the lophophorates.
1. tentaculophores
2. phoronida
3. bryozoan
4. brachiopod
5. lophophore

Mollusca, Annelida, Onychophora, and Arthropoda

The phylum that
includes the gastropods, bivalves, cephalopods, and amphineurans is termed ______________.
1. Gastropoda
2. Bivalvia
3. Amphieura
4. Cephalopoda
5. Mollusca
The phylum including the earthworms, polychaetes, and leeches is called ______________.
1. Oligochaeta
2. Polychaeta
3. Hirudinea
4. Annelida
5. Nematoda
Earthworms maintain water and salt regulation with many kidney-like structures called the ______________.
1. flame cells
2. metanephros
3. nephridia
4. trachea
5. ureters
Crustaceans, spiders, mites, ticks, and insects are all members of the phylum ______________.
1. Echinodermata
2. Onychophora
3. Arthropoda
4. Arachnida
5. Insecta
Lobsters, shrimp, crabs, crayfish, barnacles, wood lice, pill bugs, and water fleas are all members of the ______________ class of organisms.
1. Echinodermata
2. Onychophora
3. Crustacea
4. Chilopoda
5. Diplopoda
Arthropods have specialized excretory structures that accomplish functions analogous to those carried out by the kidneys in vertebrates. These arthropod structures are called ______________.
1. kidneys
2. spiracles
3. Malpighian tubules
4. metanephros
5. flame cells
Insects have tiny pores called ______________ that allow air to pass from outside their cuticle to the interior parts of their body.
1. spiracles
2. Malpighian tubules
3. spicules
4. blastopores
5. gastropores
The centipedes are
members of the arthropod class ______________.
1. Chilopoda
2. Diplopoda
3. Crustacea
4. Bivalvia
5. Onychophora
The millipedes are members of the arthropod class ______________.
1. Chilopoda
2. Diplopoda
3. Crustacea
4. Bivalvia
5. Onychophora

Echinodermata and Hemichordata

Which of the following groups of organisms has pentaradial symmetry?
1. crinoids
2. starfish
3. sea urchins
4. sea cucumbers
5. all of the above
Without the following, it wouldn't be possible for the tube feet or podia, which enable echinoderms to move about, to function: ______________.
1. water vascular system
2. hydraulic pressure
3. fluid-filled vessels
4. all of the above
5. none of the above
Sea lilies and feather stars have branched, feathery arms that use their mucus to trap food particles floating by. These forms are members of which echinoderm class?
1. Crinoidea
2. Asteroidea
3. Ophiuroidea
4. Echinoidea
5. Holothurioidea
Starfish, or sea stars, are members of which echinoderm class?
1. Crinoidea
2. Asteroidea
3. Ophiuroidea
4. Echinoidea
5. Holothurioidea
A marine phylum that contains about 85 species that possess a series of gill slits along the wall of their pharynx, as well as having ciliated larvae, is known as ______________.
1. Echinodermata
2. Chordata
3. Hemichordata
4. Annelida
5. Arthropoda

Chordata

The organisms
that at some stage in their life history contain a notocord, a dorsal hollow nerve cord, and pharyngeal gill slits are known as ______________
1. Urochordata
2. Cephalochordata
3. Vertebrata
4. Chordata
5. all of the above
Tunicates and sea squirts are members of one of the chordate subphyla known as ______________.
1. Urochordata
2. Cephalochordata
3. Hemichordata
4. Vertebrata
5. Chordata
Unlike the ______________ and the ______________, the vertebrates have a vertebral column.
1. Urochordata, Cephalochordata
2. Agnatha, Chondrichthyes
3. Osteichthyes, Aves
4. lampreys, hagfish
5. all of the above
What group of chordates that are evolutionarily more advanced than the fish have fish-like eggs and are external fertilizers?
1. amphibians
2. reptiles
3. birds
4. mammals
5. ostracoderms
The characteristics that enabled ______________ to become so successful during the Mesozoic era included their shelled eggs and internal fertilization.
1. amphibians
2. reptiles
3. birds
4. mammals
5. fish
Unlike modern birds, ______________ had teeth and a long bony tail, but they also had feathers.
1. Archaeopteryx
2. early amphibians
3. early reptiles
4. early mammals
5. none of the above
Besides modern
birds, the following group of chordates also has a four-chambered heart:
1. fish
2. amphibians
3. reptiles
4. mammals
5. hemichordates
Like the birds, mammals are ______________.
1. homeothermic
2. poikilothermic
3. amniotes
4. a and b
5. a and c
After birth, mammals nourish their young with milk that is secreted from the ______________ of the mother.
1. hair
2. mammary glands
3. sebaceous glands
4. salivary glands
5. none of the above

ANSWERS

Questions to Think About

What features do members of the animal kingdom share?
What are all the different kingdoms of organisms, and what features characterize each one?
What differentiates members of the animal kingdom from the other kingdoms?
List as many phyla within the kingdom Animalia as you can think of, and then write down their main characteristics.
What differentiates the protostomes from the deuterostomes?
Are humans more closely related to starfish or to insects? Why?
Compare and contrast the arthropods and the mollusks. Include major groups of each phylum.
What features do all chordates have in common? Do they share any of these with a possible common ancestor? If so, what is that common ancestor thought to be? Describe a living relative.
What characteristics are used to classify the major groups (classes) of vertebrates? Use those characteristics to describe each of the vertebrate classes.