BIOL 1002 Chapter : Invertebrate Animals Part 2 Of 3
Document Summary
All the remaining phyla in the kingdom animalia have bilateral symmetry and three germ layers. Some species (sea urchins and starfish) have radial symmetry as adults but have bilateral symmetry in the larval stages. The embryo forms of all the phyla we have discussed so far, as well as the protostomes, go through spiral/determinate cleavage. By the time they get to about the four-cell stage the cells have lost their totipotency and their fate is already fixed. The embryos of the deuterostomes go through radial/indeterminate cleavage. The cells present after the four-cell stage retain their totipotency (stem cells) and the ability to develop into any type of cell or even a complete embryo. Platyhelminthes, annelida, mollusca (flat worms, segmented worms and mollusks) Flat shape, many are parasitic (on left), some are free living (2 on right), most are hermaphroditic, can self-fertilize.
