vii PHYLUM ECHINODERMATA 163 



This radial blood-vessel, as it is termed, is divided lon- 

 gitudinally by a vertical septum into two lateral halves. 

 Internally it communicates with an oral ring-vessel surround- 

 ing the mouth and likewise divided into two by a septum. 

 When the dorsal wall of the central disc is dissected away, 

 the remainder of the organs come into view. The rows of 

 ambulacral ossicles appear on this view as ridges, the 

 ambulacral ridges, one running along the middle of the 

 ventral surface of each arm to its extremity, and extending 

 inwards to the corresponding angle of the mouth. At the 

 sides of each of these ridges appear the rows of ampulla?. 

 Within the pentagonal actinostome is a space, the peristome, 

 covered with a soft integument, and in the centre of this is a 

 circular opening, the true mouth, the size of which is capable 

 of being greatly increased or diminished. 



The mouth is found to open through a short passage, the 

 oesophagus, into a wide sac, the cardiac division of the stomach 

 (Fig. 88, st; Fig. 89, card. si). This is a five-lobed sac, 

 each of the lobes of which is opposite one of the five arms. 

 The walls of the sac are greatly folded, and the whole is 

 capable of being everted through the opening of the mouth, 

 folded over some object desired as food, and then retracted 

 into the interior, the retraction being effected by means of 

 special retractor muscles which arise from the sides of the 

 ambulacral ridges. This cardiac division of the stomach 

 communicates dorsally with a much smaller chamber, the 

 pyloric division of the stomach, and this in turn opens into a 

 very short conical intestine, which leads directly upwards to 

 open at the small anal aperture. The pyloric division of 

 the stomach is pentagonal, each angle being drawn out to 

 form a pair of large, tree-like appendages, the pyloric caca 

 (Figs. 88 and 89, pyl. ccec), which extend to near the 

 extremity of the arm. The walls of the pyloric caeca are 



