362 



ECHINODERMATA. 



ferior surface of the body. On the other hand, in the stalked Crinoids 

 and certain other forms, the ventral surface is directed upwards, and 

 the mouth is thus placed on the superior aspect of the body. 



Among the most characteristic of all Echinodermal structures are 

 the so-called " water-vessels " or " ambulacral " vessels. This system 

 consists of a series of musculo-membranous tubes filled with a watery 

 fluid, and connected with the function of respiration, while at the 



same time commonly subserving 

 locomotion. It consists essen- 

 tially (fig. 235) of a circular 

 vessel which surrounds the com- 

 mencement of the alimentary 

 canal, and gives off secondary 

 vessels in a radiating manner. 

 The " radiating vessels " usually 

 give off at right angles numerous 

 short lateral tubes (the "tube- 

 feet " or " pedicels ") ; and the 

 " circular vessel " is generally 

 placed in communication with 

 the exterior by a special canal 

 (" stone-canal " or " water-tube ") 

 which opens on the surface by 

 a spongy calcareous plate ("ma- 

 dreporite "). Though commonly 

 subserving locomotion, the am- 

 bulacral vessels are probably 

 primarily respiratory in function ; 

 and they not uncommonly give 

 off leaf-like or branched external 

 processes ("ambulacral gills"), 

 which serve as respiratory or- 

 gans. There is also a second 

 vascular system, consisting of an 

 oral ring and radial trunks, which 

 are situated between the water- 

 vessels and the nerve -bands. 

 The oral ring is in relation with 

 a peculiar organ, formerly de- 

 scribed as a heart, but probably to be regarded as a kidney, which 

 in many Echinoderms seems to communicate with the exterior by 

 means of the madreporite. 



The sexes are almost always distinct in the Echinoderms, and the 

 reproductive organs are usually lodged in the interior of the body, 

 definite openings for the escape of the generative elements existing 



Fig. 235. — Diagram of tne ambulacral system 

 of Echinus, m, Madreporite ; s, Stone-canal ; 

 r, Central oesophageal ring ; p p, Polian 

 vesicles ; a a, Radiating ambulacral vessels. 

 Only the bases of four of the radiating ves- 

 sels are shown ; and a few of the tube-feet {£), 

 with their secondary vesicles or "ampullae" (v), 

 are shown on one side of one of the radiating 

 canals. 



