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.
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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.