566 EC?IINODERMATA HOLOTHUROIDEA chap. 



same time. This extraordinary modification is the consequence 

 of the habit of forcing water into the respiratory trees. The 

 body-cavity is by this means kept tensely filled with fluid, and 

 the stone-canal is enabled to draw on it for the supply to the 

 water-vascular system, thus rendering the external madreporite 

 supererogatory. A large-stalked sac — the Polian vesicle (Fig. 

 256, 4) — multiplied in many species, hangs down from the 

 water-vascular ring and serves as a reservoir of fluid. 



All the podia, including the feelers, have ampullae. In the 

 feelers a semicircular valve is situated just where the external 

 part passes into its long ampulla. When this valve is expanded, 

 the feeler is moved about by the contraction of its muscles, but 

 when it is contracted, the contents of the feeler can flow back 

 into the ampulla, so that the feeler is reduced to an insignificant 

 papilla (as in Fig. 254). The interior of the feeler is ciliated, 

 and a current seems to flow up one side and down the other, so 

 that this organ, like the dorsal tube-foot of a Cake-urchin or 

 Heart-urchin, seems to assist in respiration. 



The nervous system differs from that of Echinoidea in the 

 absence of the pigment spot (or so-called eye) on the terminal 

 podium of the radial water -vascular canal. Each podium 

 receives a so-called nerve — really an extension of the radial 

 nerve-cord with its ganglion -cells — and this ends in a plate of 

 sensory epithelium in the sucker of the tube-foot or tip of the 

 tentacle, or of each of its branches in the case of the feeler. 



There is a coelomic nervous system developed from the 

 radial perihaemal canals. The perihaemal ring is represented 

 in Echinoidea by the lantern coelom, in Holothuroidea in all 

 probability by the " buccal sinus," a space intervening between 

 the water-vascular ring and the oesophagus. In the outer wall 

 of this are developed ossicles, which constitute the calcareous 

 ring found in all-' Sea-cucumbers (Fig. 257, A and B). In this 

 ring (Fig. 257, B) are to be distinguished radial and interradial 

 pieces. The former are notched at their upper ends, and in all 

 probability represent the auriculae of Echinoidea, as the radial 

 nerve-cords pass out over the notches, whilst the interradial 

 pieces probably represent a coalesced pair of jaws and their 

 included tooth, since these ossicles develop from a single 

 rudiment in the larval Echinoid. 



^ Pelagothuria is said to liave 110 calcifications. 



