382 



ZOOLOGY 



to this visceral layer of the peritoneum, the wall of the ali- 

 mentary canal and its caeca consists of a muscular layer and an 

 internal lining, the enteric epithelium or endoderm (Ent. Epthm). 

 The ccelome is filled with a fluid, the coslomic fluid, consisting 

 mainly of sea-water, but containing a number of amoeboid cor- 

 puscles (amasbocytes) containing a brown pigment. The dermal 

 branchiae consist of a muscular layer, an external epidermal layer, 

 and an internal peritoneal layer, the internal cavities of the hollow 

 branchiae being in free communication with the ccelome. 



Digestive System. — The mouth is found to open through a ' 

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

 of the stomach (Fig. 305, St, Figs. 308, 310, 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 cap- 

 able of being everted 

 through the opening of 

 the mouth,wrapped over 

 '^^^^M^^^M^^^^WfM^i^^ some object desired as 



food, and then retracted 

 into the interior, the re- 

 traction being effected 

 by means of special 

 retractcrr muscles (Fig. 

 308, retr) which arise 

 from the sides of the 

 ambulacral ridges. This 

 cardiac division of the 

 stomach communicates 

 aborally with a much 

 smaller chamber, the 

 pyloric division of the 

 stomach, and this in turn opens into a very short conical in- 

 testine, which leads directly upwards 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 appendages, the pyloric 

 cmca (Figs. 305, 306, 308, 310, pyl. case). Each pair of pyloric 

 caeca 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, extending 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 



Figs. 306 

 anus 

 ccec. 



Asterias rubens. Digestive system. 



; card. st. cardiac division of the stomach ; 



intestinal cmca ; madr. madreporite ; pyl. 



pyloric cseca ; pyl. st. 

 (From Leuckart.) 



pyloric division of the stomach. 



