IX 



PHYLUM ECHINODERMATA 



Digestive System. The mouth is found to open through a 

 short passage, the oesophagus, into a wide sac, the cardiac division 

 of the stomach (Fig. 280, St, Figs. 282, 284,_ 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, 

 wrapped over some object desired as food, and then retracted into 

 the interior, the retraction being effected by means of special 

 retractor muscles (Fig. 283, retr) which arise from the sides of 

 the ambulacral ridges. This cardiac division of the stomach com- 

 municates dorsally with a much smaller chamber, the pyloric 

 division of the stomach, 

 and this in turn opens 

 into a very short coni- 

 cal intestine, which leads 

 directly upwards to 

 open at the anal aper- 

 ture. The pyloric divi- 

 sion of the stomach is 

 pentagonal, each angle 

 being drawn out to 

 form a pair of large 

 appendages, the pyloric 

 cceca (Figs.' 280, 281, 

 283, and 284, pyl ccec\ 

 Each pair of pyloric 

 cseca commences as a 

 cylindrical canal or 

 duct, the lumen of 

 which is continuous 

 with the cavity of the 

 pyloric chamber. This 

 soon bifurcates to form 

 two hollow stems, ex- 

 tending to near the extremity of the cavity of the arm, and giving 

 off laterally two series of short branches, each having connected 

 with it a number of small bladder-like pouches. The walls of the 

 pyloric caeca are glandular : they secrete a digestive fluid, and are 

 therefore to be looked upon as digestive glands. It is found by 

 experimenting with this digestive fluid that it has an action on 

 food-matters similar to that exerted by the secretion of the 

 pancreas in the Vertebrata, converting starch into sugar, proteids 

 into peptones, and bringing about the emulsification of fats. 

 While the pouches of the cardia^jliyigionof the stomach are 

 attached to the ventral^jxaJl^Tthe bod^jblie pyloric creca are 

 connected with the dorsalwair From the short intestine are 

 given off inter-radially two hollow appendages, the intestinal caca 



VOL. I A A 



FIG. 281. Asterias rub ens. Digestive system; an, 

 anus; m/v/. xt. cardiac division of. the stomach; int. 

 (!. intestinal cteca ; ma</r. madreporite ; pyl. c<c. 

 jiylnric cteca ; pt/l. st. pyloric division of the stomach. 

 (From Leuckart.) 



