Invertebrate Gallery of the Indian Museum. 97 



thickly covered with stout calcareous spines, and some 

 of these are often modified to form beak-like or pincer-like 

 organs known as " pedicellariae," examples of which are 

 shown in Wall-case 18. 



The Echinodermata are specially characterized by the 

 possession of a "water-vascular system/' by which a 

 watery fluid is distributed through the body, assisting 

 respiration and the excretion of waste products and aid- 

 ing in locomotion. This system is also in communication 

 with the external sea-water. 



The details of the water-vascular system, which are 

 shown diagrammatically in the enlarged drawing in Case 18 

 and are also displayed in several dissections, are typically 

 as follows : — 



Through a sieve-like usually external plate, known as 

 the " madreporic plate," the sea-water percolates and 

 passes along a canal, known as the *' stone-canal," into a 

 circular canal which surrounds the gullet. On this "circular 

 canal " there are usually found several reservoirs known as 

 " Polian vesicles, " and from the circular canal five radial or 

 "ambulacral" canals branch off, one into each ray of the 

 body. From both sides of the ambulacral canals erectile 

 tubes are given off, and these, which are known as " am- 

 bulacral feet " or "tube feet," pass through pores in the 

 bcdy-wall and project outside the body. The tube-feet 

 commonly end in a sucker, and it is by the contraction of 

 the turgid tube-feet, when the suckers are firmly adherent 

 to some external object, that the typical Echinoderm 

 travels. Sometimes the water- vascular system is prolong- 

 ed into branching tentacles which surround the mouth, 

 as is seen in the Holothurian, Thyone, exhibited in 

 Case 35. 



The alimentary canal differs much in the different 

 groups of the Echinodermata : its details in several of 

 the groups are shown by dissections with explanatory 

 notes. In some groups the mouth is armed with powerful 

 teeth, specimens of which are exhibited (Cases 28-29). 



