viu ECHINODERMATA ALIMENTARY CANAL 483 



is occasionally somewhat widened, and is then called the stomach. 

 In Rhizocrinus and Bathycrinus there are, on the external side of the 

 digestive tract, interradially placed outgrowths. Similar outgrowths 

 occur in great numbers on the inner side of the tract in Antedon (the 

 side facing the axis of the calyx). Such a diverticulum, when espe- 

 cially large and branched, has been called a hepatic caecum, but this 

 name must not be accepted in any strict sense. 



The finer structure of the intestine agrees in essentials with that 

 in other Echinoderms. The intestinal epithelium is everywhere ciliated 

 except in part of the rectum. The musculature is weakly developed 

 or altogether wanting, except near the mouth and in the rectum, 

 where sphincters are formed. The anal tube or cone consists of the 

 body wall externally and of the wall of the rectum internally. 

 Between these two the reduced body cavity is traversed by radially 

 placed strands of connective tissue. 



E. Asteroidea (Fig. 385). 



That part of the oral area which is left free by the skeleton is 

 covered by a soft oral integument, in the middle of which lies the 

 mouth. This organ can be opened by muscles which run out from it 

 radially in the oral integument ; it can be closed by circular muscle 

 fibres, which run round it in the oral integument and in the 

 oesophagus. 



The mouth leads into an oesophagus, which ascends perpendicu- 

 larly, widens rapidly, and passes over without any sharp boundary 

 into the stomachal sac. 



In Echinaster scpositus, the oesophagus has around it ten outgrowths, whose 

 walls are very much folded, and whose (inner) epithelium is richly supplied with 

 glands. 



The membranous stomachal sac of the Asteroids is very spacious, 

 filling the whole disc. Its wall is irregularly folded, and provided 

 with outgrowths ; it is connected with the wall of the disc by means 

 of mesenterial strands, partly of connective tissue, partly of muscle. 



In the upper (apical) portion of the stomachal sac, five pairs of 

 braehial divertieula open ; these are the radial caeca, or hepatic 

 appendages, which stretch more or less far into the arms, according 

 to family, genus, or species. There is one pair in each arm. These 

 divertieula of the stomachal sac (which, ontogenetically, develop com- 

 paratively late) have the following general structure. Each diver- 

 ticulum consists of a median common tube, which runs in the longi- 

 tudinal direction of the arm, giving off lateral tubes alternately to 

 right and left. Each lateral tube receives from each side the openings 

 of closely crowded glandular lobes, so that the secreting surface is 

 very large. 



