a. Animals are placed in the kingdom Animalia, or Metazoa.

b. Animals are multicellular holotrophs that obtain nutrients by engulfing and digesting food.

c. Most animals are diploid.

d. Most animals reproduce sexually with eggs and sperm that form a zygote, then an embryo, which either grows into an adult, or becomes a larva, an immature form very different from the adult.

a. Animals and their evolutionary history are best described with regard to their body plans and their embryology (Concepts 34.1, Figure 34.1).

b. The differences among the earliest metazoan phyla are so large that it is difficult to determine their common ancestor, yet good arguments propose that the kingdom Metazoa is monophyletic (Figure 34.25).

c. Table 34.1 shows a classification of the animal kingdom.

d. Various authorities have divided the animals into some 20 to 40 phyla.

e. Some primitive animals are set off into subkingdoms:

f. The Eumetazoa ("true animals") are divided into two infrakingdoms, Radiata and Bilateria, on the basis of their body symmetry and number of tissue layers.

g. The Radiata (e.g. corals, sea anemones, and jellyfish) are radially symmetrical and diploblastic, with a jellylike material between their two tissue layers.

h. The Bilateria are all other animals, which are mostly bilaterally symmetric and triploblastic (having three tissue layers.).

i. The Bilateria are divided into three groups on the basis of their body plan and with respect to the coelom, a cavity between the body wall and the intestine.

a. The origin of the animal kingdom is a subject of debate, though it is generally agreed that the sponges arose from ancestors different from those of the eumetazoans.

b. Concepts 34.2 explains how an animal's embryonic development holds clues to the forms of its ancestors.

c. Zoologists have theorized that an early hollow blastea form in evolution was followed by a gastrea form, as a parallel to the blastula and gastrula stages in embryological development.

d. The larval stage of the cnidarians is a flattened, hollow, two-layered planula, and sponges have a flat, one-layered larva.

e. Most theorists agree that the ancestral metazoan was a flattened, colonial flagellate that evolved into a hollow, two-layered form and then perhaps into something like a gastrea, with an invagination that formed a simple digestive pouch.

f. Otto Bütschli proposed that the primitive eumetazoan was something like a planula, and he termed it a plakula (Figure 34.3), a bilaterally symmetrical animal with two cell layers that ingested food with its bottom layer.

g. Bütschli theorized that this animal raised itself up into a temporary chamber for feeding, and thus provided a reason for a gastrula-like form.

h. Karl Grell rediscovered Trichoplax in 1974 and showed that it is not a larva, but that it does form a temporary digestive cavity, that its symmetry could have been ancestral to both bilateral and radial forms, and that it reproduces sexually.

a. There are about 10,000 species of sponges, ranging widely in size and color, but all modular animals are made of small units that usually reproduce asexually.

b. Sponges are filter feeders that pump water through incurrent pores, through a central cavity, and out an osculum (Figure 34.4).

c. Sponges produce a water current with flagellated collar cells, or choanocytes (Figure 34.5), which trap food particles and either digest them or move them to other cells for that purpose.

d. There are three basic body types of sponges: Asconoid, Syconoid, and Leuconoid (Figure 34.6), which vary with regard to the complexity of their body plans.

e. Choanocytes are a distinctive sponge feature, and they connect the sponges to the colonial flagellates (Figure 34.7), their probable ancestors.

f. Sponges are aggregates of cells without true tissues, but with some differentiation:

g. Sponges are supported by a skeleton made of spicules, which are composed either of calcium carbonate or silica (Figures 34.8, 34.9).

h. Sponges are diploid and reproduce sexually; most are hermaphroditic but produce sperm and eggs at different times.

i. Some sponges can reproduce asexually, as groups of cells break off, attach somewhere else, and continue to grow.

j. In adverse conditions, some sponges form gemmules, which are special clusters of cells that are resistant to drying and that can develop into new sponges in favorable conditions.

a. The cnidarians are mostly marine animals that are radially symmetrical with either sessile, polyp bodies or floating bowl-like shapes called medusas (Figure 34.10).

b. Cnidarians have a coelenteron (the phylum was formerly named Coelenterata), or central gastrovascular cavity that serves for both digestion and circulation.

c. The main body axis of a cnidarian extends from its oral surface at the mouth to the opposite aboral surface.

d. Cnidarians are armed with cnidocytes (from which the current phylum name is derived) which contain stinging nematocysts that bear sticky coverings, barbs, poison, or all three of these (Figure 34.11).

e. Cnidarians have ectoderm, endoderm, and a jellylike mesoglea in between.

f. Cnidarians have both epitheliomuscular cells that form an epithelium-like surface, and a nerve net that connects the tentacles and mouth (without a central brain site).

g. Cnidarians are divided into three classes largely based on their body forms:

h. All cnidarians have a sexual stage, producing sperm and eggs and a zygote that develops into a planula larva (Figure 34.12).

i. In anthozoans and Hydra, the polyp reproduces sexually, but in many other groups, the polyp stage buds into a colony or buds off medusas.

j. Members of the phylum Ctenophora are comb jellies that are named for their eight ciliated, comblike plates (Figure 34.13).

k. Ctenophora lack cnidocytes, may display luminescence, and are biradially symmetric, probably having evolved from bilaterally symmetrical ancestors.

a. The phylum Platyhelminthes contains the flatworms, which have bilateral symmetry, a definite head and tail, and three germ layers, of which the middle layer is undifferentiated mesenchyme.

b. The flatworms are acoelomate.

c. The flatworms have an incomplete digestive tract, with a mouth but no anus, and they have no circulatory system.

d. The Platyhelminthes are divided into three classes: the Turbellaria, the Cestoda and the Trematoda.

e. The Turbellaria are free-living flatworms that live in both fresh water and salt water.

f. The Trematoda are parasitic flukes.

g. The Cestoda are strictly parasitic tapeworms.

a. The phylum Nemertea contains the mostly marine ribbon worms, which are usually only a few centimeters long (Figure 34.18).

b. The ribbon worms resemble flatworms, except that they have a complete digestive tract and a closed circulatory system.

c. Phylum Nemertea has also been named phylum Rhyunchocoela, which refers to the long "hollow beak" of the proboscis worms.

d. The proboscis secretes a sticky mucus and is sometimes tipped with a hard barb and poison glands, as it is used to capture and kill prey.

a. Each of the major developments in animal structure allowed a particular group of animals to move into a new adaptive zone.

b. In the early oceanic environment of animal evolution, there were two ways of living: either epifaunal, on the surface of the bottom, or infaunal, within the mud and sand of the bottom.

c. The early cephalized, bilaterally symmetric animals were largely epifaunal, and in contact with the interstitial zone, though they were too large to enter this zone between the sand grains.

d. The successors of these animals were small turbellarian flatworms that were restricted to crawling over the ocean floor, as they were not powerful enough to burrow in the interstitial zone.

e. The evolution of the coelom and hydrostatic skeleton was a major innovation in locomotion.

f. Both circular muscles and longitudinal muscles can squeeze against the body fluids inside a coelom, resulting in stretching, shortening, and thickening of the body as the animal moves.

g. A coelom can also be used as a reservoir for excreted metabolic wastes; nephridia evolved for removing wastes from the coelomic fluid to the outside.

h. More freedom of movement is allowed with a coelom, as internal organs can be suspended and cushioned, rather than crushed or pressed into tissues.

i. With space available for pumping hearts that would not be damaged during movement, larger coelomic animals evolved circulatory systems.

j. Figure 34.19 shows a variety of animals that evolved a head end that can be extended by increasing pressure in the coelom.

k. Such an extendible unit can be used for feeding, attaching to a host, or for moving quickly, as in the case of the sipunculid worm (Figure 34.20).

a. Coeloms have probably evolved independently at least three times in different branches of the animal family tree.

b. The gonocoel theory proposes that the coelom developed through enlargement of the gonadal cavities of acoelomates (Figure 34.21).

c. This theory follows the development of the coelomoduct, which provided an outlet for both gametes and wastes that accumulate in the coelom.

d. The gonocoel theory could explain why gonads (e.g. mammalian ovaries) so often arise from the coelom wall and release their gametes into the coelom.

e. The ectoderm-derived coelomoduct is distinguished from the mesoderm-derived nephridium on the basis of their embryological origins.

f. The flatworm protonephridium ends in closed sacs containing flame cells (Figure 34.22), but more complex animals have a comparable metanephridium with a funnel-shaped opening that conducts coelomic fluid into the tubule (Figure 34.23).

g. The enterocoel theory originated in 1877 with E. Ray Lankester, who observed that the coelom on many animals actually originates through outpockets of the archenteron.

h. Lankester proposed that embryological development is recapitulating evolution in the case of the coelom.

i. This theory proposes that modified pouches develop from the gut and explains why the animals to which it applies commonly have three-part coelomic cavities (Figure 34.23).

a. The diversity of animals can be understood by focusing on their embryology in comparison with their adult body plans.

b. Most eumetazoans are based on tube-within-a-tube body plans, where the coelom occupies the space between the tubes.

c. The roundworms and their allies are pseudocoelomates, with a coelom that is formed from the former blastocoel and lined with mesoderm along only one side.

d. All other bilaterians are coelomate, with "true" coeloms that are completely lined by a layer of mesoderm called the peritoneum.

e. The peritoneum-forming mesoderm forms sheets on each side of the coelom that meet in the center to form the mesentery, a double layer of tissue that surrounds and anchors the internal organs.

f. Coelomate animals are divided into the Protostomia (protostomes with the mouth arising from the blastopore) and Deuterostomia (deuterostomes with the anus arising from the blastopore).

g. Protostome embryos generally divide by spiral cleavage and have mosaic development.

h. Deuterostome embryos divide by radial cleavage and have regulative development.

i. In animals that pass through a free-living larval stage, the protostome larva is typically a trochophore, and the deuterostome larva is a dipleurula (Figure 34.24).

j. The protostome coelom is schizocoel, formed by a single blastomere (the mesentoblast) that divides to produce the entire mesoderm.

k. The deuterostome coelom is enterocoel, formed by a pocket of cells that pouch out of the primitive gut and close into hollow masses.

l. These embryological considerations, taken together, indicate that coelomic animals belong to two main clades (Figure 34.25).

a. Of the several animals that have pseudocoeloms, the rotifers and nematodes deserve further attention.

b. Roundworms, or nematodes, are often minute and hairlike, and are extremely abundant creatures, with 10,000 species described so far and numerous individuals per species living on the earth (Figure 34.26).

c. The roundworm body has a tough external cuticle over a body wall of longitudinal muscles; they commonly move by "thrashing," as they have no circular muscles.

d. Roundworms have single dorsal and ventral nerve cords, an excretory system, but no circulatory system.

e. Ascaris lumbricoides is a common nematode that is an intestinal parasite (Figure 34.26).

f. Many parasitic nematodes have durable eggs that, once consumed, can hatch and infect a new host.

g. Trichinosis is a nematode disease that is caused by the minute Trichinella spiralis, which moves from the intestine through the circulatory system and into the muscles of its hosts (Figure 34.27).

h. Rotifers (phylum Rotifera) are abundant in freshwater habitats.

i. These "wheel animals" have a ciliated corona on their anterior ends (Figure 34.28).

j. Rotifers have an excretory system with flame cells, a small concentration of neurons that form a brain, but no circulatory system.

k. Some rotifer species consist exclusively of females that reproduce parthenogenetically; other species produce males only at certain times of the year.

l. Many rotifers can encase themselves in a gelatinous envelope for protection against dehydration and temperature stresses.