ECHINODERMA. ASTEROTDEA 



109 



along the arms, and on the aboral surface of the disc there 

 is sometimes a central plate with others arranged in circles 

 around it. Short spines are often abundant. Pedicellarice 

 are also of frequent occurrence, especially on the aboral 

 surface ; they are modified spines consisting typically of 

 two movable blades which are often borne on a stalk, and 

 serve to remove foreign bodies from the surface of the 

 animal. Projecting into the mouth at the angles formed 

 by the ambulacral grooves are five pairs of plates, which 

 give the mouth a star-shaped form. The mouth leads 

 into a short oesophagus which opens into the globular 

 stomach ; above the stomach is the pentagonal pyloric 

 sac, from the angles of which are given off branches, which 

 soon divide into two, and ex- 

 tend down the arms near the 

 aboral surface. From the 

 pyloric sac a short narrow 

 intestine leads to the anus 

 at the centre of the aboral 

 surface. The water- vascular 

 system consists of a vessel 

 forming a ring round the 

 oesophagus (fig. 43, a), from 

 which ring a branch (6) is 

 given off to each arm and is 

 placed in the ambulacral 

 groove outside the skeleton. 

 Each of these radial vessels Fi g- 43 - Diagram of the water 



_ vascular system of a star-fish 



gives Oft two TOWS Ot processes a , circular vessel round the 



on opposite sides, which pass 

 through the pores between 

 the ambulacral ossicles into 

 vesicles situated above the ossicles and known as the 



mouth ; b, radial vessels ; c, 

 Polian vesicles ; d, stone-canal ; 

 e, madreporic plate ; /, tube-feet 

 (only a few shown) ; g, ampulla. 



