vir PHYLUM ECHINODERMATA 163 



oral ring-vessel, surrounding 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 

 proximally to the corresponding angle of the mouth. At 

 the sides of each of these ridges appear the rows of ampullae. 

 Within the pentagonal actinostome is a space, \he peristome, 

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

 circular opening, the true mouth (Fig. 82, Mth.), 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. 82, st., Fig. 83, card. st.\ 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 (Fig. 83, pyl. st.\ 

 and this, in turn, opens into a very short conical intestine, 

 which leads directly upwaids to open at the 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 cceca (Figs. 82 and 83, pyl. cac.}, which extend 

 to near the extremity of the arm. The walls of the pyloric 

 caeca are glandular : they secrete a digestive fluid, 

 and are therefore to be looked upon as digestive glands. 



M 2 



