5 I 6 ECniNODERMATA ECHINOIDEA chap. 



The internal organs of the Urchin can best be examined by 

 making a horizontal incision about one- third the distance from 

 the mouth and pulling the two parts gently asunder. A large 

 amount of fluid escapes from the exceedingly spacious coelomic 

 cavity, the alimentary canal being comparatively narrow. 



The alimentary canal commences with a short vertical tube 

 which has been shown to be a stomodaeum ; this is surrounded by 

 the upper ends of the teeth and their supporting ossicles, which are 

 collectively termed " Aristotle's lantern." The oesophagus leads 

 into a baggy, flattened tube, the stomach, which runs horizontally 

 round the animal, supported by strings of tissue from the 

 coelomic wall, so that it hangs down in a series of festoons. 

 Having encircled the animal, it bends directly back on itself and 

 immediately opens into the intestine, which is also a flattened 

 tube, which runs round the circumference of the animal, but in 

 the opposite direction, the festoons of the second circle alternat- 

 ing with those of the first. The intestine opens into a short 

 rectum which ascends vertically to open by the anus. The 

 stomach is accompanied by a small cylindrical tube called the 

 "siphon" (Fig. 231, sij^h), which opens into it at both ends ; this 

 represents merely a gutter which has been completely grooved 

 off from the main intestine ; it is lined by cilia, and its function 

 is believed to be that of keeping a stream of fresh water flowing 

 through the gut, so as to subserve respiration. 



UcMnus esculentus seems to feed chiefly on the brown fronds 

 of Laminaria and the small animals found thereon, which it 

 chews up with its teeth, but it may regale itself on the same diet 

 as Brittle Stars, as Allen ^ has shown to be the case in Plymouth 

 Sound. Dohrn ^ has described the Neapolitan Sphaerecliinus 

 granulans attacking and capturing Crustacea such as Squilla. 



The water -vascular system presents several features of 

 great interest. The ring-canal is situated at a considerable 

 distance above the nerve-ring, and is separated from it by the 

 whole of the jaws and teeth. It has five small interradial 

 pouches on it, which apparently correspond to Tiedemann's 

 bodies in an Asteroid. The stone -canal (Fig. 231) opens as 



^ ' The Fauna and Bottom Deposits near the thirty-fathom line from the Eddy- 

 stone grounds to Startpoint," Journ. Marine Biol. Ass. v. 1899, p. 472. 



^ "Mitth. uber die zool. Stat. v. Neapel," Zeitschr. wiss. Zool. xxv. 1875, p. 

 471. 



