CoachAmong coelomate animals, the fate of the first opening of the embryonic gut divides two great lineages: in protostomes (annelids, molluscs, arthropods) it becomes the mouth, the coelom forms by splitting of the mesoderm, and the early cleavages of the egg are spiral and their fate fixed early; in deuterostomes (echinoderms, vertebrates) it becomes the anus, the coelom forms by outpocketing of the gut, and the early cells remain flexible in fate.
Examples
Example 4.6 (Reading an animal from its plan)
An animal with a head end, a through gut, a body of similar rings and no hard parts: a coelomate protostome moving by a hydrostatic skeleton, each segment’s coelomic fluid pressurised by circular and longitudinal muscles in turn — an earthworm burrowing. An animal with a jointed cuticle and specialised segments: an arthropod, whose growth must pass through moults. Bilateral, segmented muscle blocks around a dorsal axis of vertebrae: a vertebrate, whose endoskeleton grows continuously.