9. An animal zygote undergoes cleavage divisions to form a blastula and then a gastrula, which has ectoderm, endoderm, and in some species, mesoderm. Those with two layers are diploblastic; with three layers, triploblastic.
10. Animals may undergo direct development or metamorphose from a larva to an adult. They reproduce sexually and sometimes asexually.
11. Two major lineages of bilaterally symmetric animals are protostomes and deuterostomes. Protostomes (Ecdysozoans and Lophotrochozoans) have spiral and determinate cleavage, and a blastopore developing into a mouth. Deuterostomes have radial and indeterminate cleavage and a blastopore developing into an anus.
12. A coelom is a body cavity lined with mesoderm. Animals may be coelomate, acoelomate, or pseudocoelomate.
20. A cnidarian body form is a polyp or a medusa. In species that alternate between these forms, fertilization in water produces a planula, which attaches and becomes a polyp that reproduces asexually to generate a strobila, which yields medusae.
24.5 Flatworms (Platyhelminthes)
21. Flatworms include planaria, flukes, and tapeworms.
28. Annelids include oligochaetes, characterized by setae (bristles) and a clitellum; polychaetes, with parapodia as appendages; and the Hirudinea (leeches) with superficial annuli within their segments.
35. Arthropods have open circulatory systems, spiracles to respire, complex nervous systems, and malpighian tubules for excretion.
36. Four subphyla include the extinct trilobites; chelicerates (horseshoe crabs and arachnids); and two subphyla of mandibulates (crustacea and urinamia). Subphylum uniramia contains the insects and myriapods.
24.10 The Echinoderms: Life Based on Five-Part Symmetry
37. Echinoderms are spiny-skinned marine animals with pentaradial symmetry that move using water vascular systems to power tube feet.