Animal Diversity
Core concepts
Organize animals by body plan: symmetry, germ layers, coelom type, segmentation, and fate of the blastopore. Know the key phyla and their diagnostic features - Porifera (choanocytes, spongin or spicules, no true tissues), Cnidaria (cnidocytes, diploblastic, radial), Platyhelminthes (acoelomate, flame cells), Nematoda (pseudocoelomate, cuticle, complete gut), Annelida (metameric, closed circulation, nephridia), Mollusca (mantle, radula, open circulation except cephalopods), Arthropoda (chitin exoskeleton, jointed appendages, tagmata, open circulation, Malpighian tubules), Echinodermata (water vascular system, pentaradial adults, deuterostome), and Chordata (notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail). Compare protostome and deuterostome development, and place invertebrate chordates correctly.
Common traps
Do not call Nematoda coelomate - it is pseudocoelomate. Echinoderm larvae are bilaterally symmetrical, so adult radial symmetry is secondary. Arthropoda has an open circulatory system; students often wrongly assign closed circulation. Sponges lack true tissues and organs. Distinguish the notochord from the vertebral column: all chordates have a notochord at some stage, not all have vertebrae.
Worked example
Rule of thumb: use a two-step key - first germ layers (diploblastic versus triploblastic), then coelom (acoelomate, pseudocoelomate, coelomate). A triploblastic, pseudocoelomate worm with a cuticle and complete digestive tract keys out to Nematoda, not Platyhelminthes, which is acoelomate and has a blind gut.