THE PHYLUM ECHINODERMATA 



and dragging itself along. By this means it can even walk up the glass side 

 of an aquarium. 



The digestive system begins as a small, muscular pharynx, surrounded by 

 the tough, ring-like structure to which the tentacles are attached, and which 

 can be retracted by the contraction of five stout muscles running to the body 

 wall (Fig. 16.13). The pharynx is followed by a short esophagus, a small 

 muscular stomach, and a long, looped intestine. The intestine traverses the 

 length of the coelomic cavity three times, supported along part of its course 

 by dorsal and ventral mesenteries containing blood vessels. At its posterior 

 end, the intestine enlarges to form the cloaca, which opens to the exterior 

 at the anus. The cloaca bears a pair of branching, tubular structures, the 

 "water lungs," which are filled with water drawn through the anus and 

 pumped into them by cloacal contractions. Through the walls of these 

 tubules respiratory exchange occurs between the water and the coelomic fluid, 

 and the water is periodically expelled by contraction of the body wall. 



Thyone shows peculiarities in other organ systems. The ambulacral system 

 consists of the same parts found in the starfish and the sea urchin, arranged 

 somewhat differently. The ampullae of the tube feet are scattered over the 

 internal surface of the body wall but connect with five radial ambulacral 

 canals. The stone canal springs from the ring canal about the pharynx and 

 ends in an internal madreporite, which hangs free in the coelomic cavity and 

 has no external openings. A so-called haemal system, very rudimentary in 

 other echinoderms, is well developed in holothurians. In Thyone it is par- 

 ticularly conspicuous in connection with the digestive tract, to which branches 

 of the haemal system run in the mesenteries. Although there is no heart or 

 other propulsive organ, the haemal system contains a fluid which probably 



Fig. 16.14. Generalized, sche- 

 matic life cycle of a holothuroid 

 echinoderm. Through inter- 

 mediate stages essentially sim- 

 ilar to those of an asteroid, the 

 gastrula (.4) transforms into an 

 auricularia larva (5); this later 

 metamorphoses into a young sea 

 cucumber (C). (Adapted from 

 H. B. Fell, 1948, Biological Re- 

 liiews, vol. 23, printed by per- 

 mission of the Cambridge 

 University Press.) 



505 



