viii ECHINODERMATA ALIMENTARY CANAL 479 



suggestion recently made that their cloacal caecum may function as a rudimentary 

 organ of this kind. 



The inner surface of the alimentary canal often shows a longitudinal fold. 

 Transverse intestinal folds, arranged in longitudinal rows, have been observed in 

 the small intestine of the Aspicl.odiirotaz. 



Finer structure of the alimentary canal. The wall of the digestive tract 

 consists of the following layers, which may vary greatly in thickness and special 

 structure in the different sections : (1) An inner intestinal epithelium ; (2) an inner 

 layer of connective tissue with the blood lacunae ; (3) a muscle layer (generally con- 

 sisting of an inner layer of longitudinal and an outer layer of circular fibres, but in 

 some Synaptidce and Aspidochirotce this order is reversed) ; (4) an outer layer of con- 

 nective tissue (often very thin) ; (5) the ciliated endothelium of the body cavity. 



The inner intestinal epithelium is ciliated, especially in the^ small intestine. 

 Over most of the digestive tract it is found as a very deep epithelium covered by a 

 fine cuticle, its cells being pallisade or thread cells. Glandular cells of various 

 sorts are specially numerous in the epithelium of the stomach. The epithelium of 

 the cloaca resembles the outer body epithelium. Into it open the processes of long 

 subepithelial glands, which are unicellular and tubular. 



The anus can be closed by means of a sphincter muscle. Calcareous plates, 

 papilla?, etc., may occur round it. 



C. Eehinoidea. 



For the position of the mouth and the anus, cf. p. 338. 



In all adult Eehinoidea, the length of the tubular intestine is 

 greater than that of a straight line from the mouth to the anus, so 

 that the course of the alimentary canal is necessarily coiled. 



The simplest arrangement is found in the Clypeastroida (Fig. 382, 

 E, p. 475). The direction of the intestinal coils will here be described 

 in the same way as in the Holothurioidea, the viscera being viewed 

 from the oral side. After traversing the masticatory apparatus, the 

 alimentary canal turns to the right (following the direction of the 

 hands of a clock), and makes rather more than a complete coil round 

 the principal axis of the body. So far, the course exactly resembles 

 that of the intestine in the Holothurioidea. In the Clypeastroida, 

 however, the canal now bends back upon itself, and runs direct back 

 to the anus, which in this division of the Eehinoidea lies orally in the 

 posterior unpaired interradius. 



In the endoeyelie or regular Eehinoidea, the arrangement is not 

 simpler, but still more complicated. After leaving the masticatory 

 apparatus, the alimentary canal ascends towards the apical system, 

 then bends round and follows the direction of the hands of a clock 

 (attached to the inner surface of the test by the mesentery) till it has 

 run about once round the principal axis. It then bends back upon 

 itself, coiling in the reverse direction backwards to the apical anus 

 (Fig. 382, D, p. 475). 



The intestine of the exocyclic Spatangoldea resembles in its course 

 that of the endoeyelie Eehinoidea with one difference, caused by the 

 facts that the mouth has shifted anteriorly along the oral surface, and 



