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Animals are multicellular eukaryotes that eat other organisms, and zoologists sort them into about 35 phyla based on body plan and DNA evidence. Key body-plan features are symmetry (none, radial or bilateral), the number of embryonic germ layers, the presence and type of body cavity, and whether the first opening of the embryo becomes the mouth (protostomes) or the anus (deuterostomes). A handful of phyla contain most familiar animals: sponges, cnidarians, flatworms, molluscs, annelids, nematodes, arthropods, echinoderms and chordates. Molecular phylogenies show that sea stars and other echinoderms are more closely related to vertebrates than to most invertebrates.
Give students specimens or photos of a sponge, a jellyfish, a planarian, a snail, an earthworm, an insect, a sea star and a fish, and have them build their own grouping from visible features. Then compare their grouping with the accepted animal tree and discuss where body features mislead, as with the radial symmetry of sea stars. A simple tree drawn on the board, with features marked at branch points, works better than a list of phyla.
There are more than a million described animal species, but most fit into a few dozen basic body plans. A phylum groups animals that share one of these plans and a common ancestry. Learning a few key features lets you place almost any animal.
Start with symmetry. Sponges (phylum Porifera) have no true tissues and no fixed symmetry. Water flows through pores in their bodies, and collar cells called choanocytes filter food from it. Cnidarians, such as jellyfish, corals and sea anemones, are radially symmetric, like a wheel. They have two tissue layers, a single opening that serves as both mouth and anus, and stinging cells called cnidocytes. Nearly all other animals are bilateral, with a left and right side, a front end and usually a head. Bilateral animals have three germ layers: ectoderm, mesoderm and endoderm.
Bilateral animals split into two great branches by how the embryo develops. In protostomes the first opening of the embryo becomes the mouth. In deuterostomes it becomes the anus, and the mouth forms later. DNA studies divide the protostomes further. One group, the Lophotrochozoa, includes flatworms (Platyhelminthes), molluscs (snails, clams and octopuses) and annelids (segmented worms such as earthworms and leeches). The other, the Ecdysozoa, includes animals that molt their outer layer: nematodes (roundworms) and arthropods. Arthropods, with jointed legs and an exoskeleton made of chitin, are the largest phylum by far. Insects alone account for more described species than all other animals combined.
The deuterostomes include two main phyla. Echinoderms, such as sea stars and sea urchins, have a water vascular system that powers tube feet. Their adults have five-part symmetry, but their larvae are bilateral. Chordates include vertebrates and a few invertebrate relatives such as lancelets and tunicates. At some point in life every chordate has a notochord, a dorsal hollow nerve cord, pharyngeal slits and a post-anal tail.
Body cavities were once used to group animals too. A coelom is a fluid-filled cavity lined by mesoderm. Older textbooks grouped animals as acoelomate, pseudocoelomate or coelomate, but DNA evidence shows these conditions arose or were lost more than once.
The pattern that emerges is a branching tree. Sponges and cnidarians branched off early. Among bilateral animals, a sea star and a human share a more recent common ancestor than either does with an earthworm or an insect.