88 ZOOLOGY. 



seems to be to drive any parrrjites away which might 

 attach themselves to the spines. 



The covering in the star-fishes is leathery, but it con- 

 tains calcareous plates, with tubercles and spines. The 

 plates, however, do not unite at the edges, as in the sea- 

 urchins. 



123. Digestion. The sea-urchins are vegetable feeders. 

 They are furnished with five long calcareous teeth, meet- 

 ing in a point. This structure has been termed Aris- 

 totle's lantern. The mouth, which is usually placed at 

 the base of the shell, leads to a gullet, stomach, and intes- 

 tine. This is much convoluted, winding round the inte- 

 rior of the shell. The anus is often, though not always, 

 placed in the apex. 



The star-fishes are animal feeders, and subsist usually 

 on bivalve mollusks. The mouth is placed in the centre 

 of the lower surface, and is destitute of teeth. The 

 stomach is prolonged into the various rays. When there 

 is an anal opening, it is situated on the upper surface. 



124. Circulation. In the sea-urchins the water -vascular 

 system communicates with the exterior by the madre- 

 poriform tubercle, situated on the largest genital plate. 

 From this tubercle proceeds a canal, which leads to a tube, 

 forming a ring round the mouth, termed the circular 

 canal. From this, five "radiating canals" proceed along 

 the ambulacral areas. These canals give off short tubes, 

 furnished with sucking discs, which are protruded through 

 apertures in the shell. They are called tube- feet, because 

 they assist in locomotion. At the base of the tube-feet 

 are situated little bladders containing water, by which 

 they are protruded or retracted at will. 



Besides the water-vascular system, there is a true blood- 

 vascular system. It consists of a central, contractile 

 cavity or heart, which gives off vessels that form rings 

 round the alimentary canal, near the mouth and anus 

 respectively. From the anal ring, five arteries proceed 

 along the ambulacral zones. These communicate with 

 five vessels which run in the opposite direction, and 



