8 



In the second or recurrent curvature the mucosa is much 

 thinner again and the epithelial cells do not have the highly 

 specialised type of those of the ,,stomach". ] ) 



In the siphon the ciliated cells do not show any special 

 granulation. 



The rectum is characterised by a thick layer of circular 

 muscles, its histology is the same as that of the rest of the gut. 



The alimentary tube of the Holothurians shows four distinct 

 parts : 1 . a short oesophagus, passing through the calcareous 

 ring (= Aristotle's lantern), frequently folded longitudinally ; 

 2. a so-called muscular stomach" (J. Miiller), very shortness 

 than 2 c.M. in Thyone), initiated by a constriction and characte- 

 rised by its musculature; 3. the so-called intestine (Frenzel 

 also calls it ,,Chylusdarm"), very long the whole gut is 

 frequently longer than 75 cM. in our small Thyone ; 4. the 

 ,,cloaca" or ,,rectum", a widened part, connected to the body- 

 wall by muscular bands transversing the coelom. The whole 

 intestine is suspended by bands of membrane, called ,,mesenteries" 

 and shows the same general curvatures and clockwise turning 

 as in all Echinoderms, as a study of cross-sections shows. Only 

 in the Synaptids it runs nearly straight from mouth to anus. 



In the stomach the food is thoroughly mixed with the visceral 

 fluid, kept there for some time and passed through in small 

 quantities. This is the case in Thyone where the food is swimming 

 about here and there in little clumps in the large quantities of 

 digestive juice. In Stichopus however, we find the whole gut 

 always full of sand, the process of digestion seems to be con- 

 tinuous here. The extreme thinness of the intestinal wall is 

 very striking and is common to many animals which eat mud 

 and sand for the sake of organic matter which they contain 

 (e. g. Sipunculus, Spatangus). Peristaltic movements, stronger 

 perhaps than those in Vertebrates, are characteristic for the 

 Holothurian intestine. 



The histology of the gut of the Holothurians has been studied 

 by Jourdan 71). Externally we find the peritoneal lining of 

 the body cavity, a pavement endothelium. It has the same two 

 types of cells which are present in the other groups. Most of 

 them are simple endothelial cells which lie in a single layer: 

 they have cylindrical shape and carry vibrating cilia. In the 

 second place we find mucus cells, which have first been described 

 by Semper. 



The second layer is the fibro-muscular one ; it has the same 

 structure as in the other groups. It contains internal longitudinal 



l ) A special gland or coecumjoccurs in the digestive tract of the Spatangoidea ; 

 it is situated on the boarder of oesophagus and first curvature. This coecum 

 contains a clear brownish yellow fluid, slightly acid, which contains proteins 

 since a precipitate is formed by heat and alcohol (3 4 c.c. ; V. Hen r y. 53)). 



