6 



in the radial sacs there is no mucle layer (C u e n o t) or a very 

 thin one (H a m a n n). The epithelium of the oesophagus shows 

 three types of cells, all of which are shorter here than in other 

 parts : a. simple epidermal cells, ressembling those of the skin, 

 b. mucus cells with one to three cilia, but without cuticula 

 and c. granular cells, with brownish or yellow grains, 

 K. C. Schneider 141) in his comparative histology of animals 

 distinguishes these three groups as ,,Nahrzellen", ,,Scheimzellen" 

 and ,,Eiweiszzellen". The cells of group a., the ,,Nahrzellen" 

 of Schneider, are ciliated (in Echinaster spinosus f.i.), they 

 are very slender and cylindrical, have a thin membrane and a 

 ,,Stiitzfibrille", connecting flagellum and nucleus. The membrane 

 forms a kind of collar, a M Kragen", sticking out from the 

 epithelium, reminding weakly of the choanocytes of the sponges. 

 They seem to play a role in resorption (C u e n o t). Those of 

 group c. have the same structure and cilia as those of group a., 

 but their cytoplasm is reticular and their secretory activity is 

 betrayed by the numerous granulae which they contain. The 

 granular cells are much longer"and much more typical in stomach 

 and radial sacs where the epithelium almost exclusively consists 

 of such >t glandular cells". Their granulae can be seen lying in 

 two or three regular lines and are very different in the different 

 stages of secretion (Schneider). 



In the sea-urchins the digestive system is altogether different. 

 The mouth is situated at the centre of a peristomial membrane 

 which forms the so-called lips. Generally we distinguish five 

 of these lying between the five teeth. These are simple folds of 

 the skin ; if the lantern is retracted they can close together so 

 as to make the whole lantern invisible. The gut commences 

 with a short vertical tube, a stomodaeum or pharynx, which 

 is surrounded by the upper ends of the teeth and their sup- 

 porting ossicles, the whole of which is called Aristotle's lantern, 

 a masticating apparatus of extreme complexity. Its mechanism 

 has been studied in detail by von Uexkiill 128). The gut 

 has pentagonal form as long as it runs through the lantern and 

 on the corners of this pentagon we find columns of connective 

 tissue. A small piece between this stomodaeum and the stomach 

 is frequently called oesophagus. It is separated from the sto- 

 modaeum by a small muscular ring which narrows the lumen at 

 that place. It is cylindrical in shape. 



The gut itself first turns around clockwise in one direction, 

 then bends sharply and comes back anti-clockwise. The first 

 part is a baggy, flat tube, sometimes called the stomach, running 

 horizontally round the animal supported by strings of tissue 

 from the coelomic wall. These mesenteries connect it at the 

 same time with the gonads, so that the whole hangs down in 

 a series of festoons, five in number. The second part, frequently 



