THE STELLEROIDEA 243 



and leads to a large stomach, which occupies most of the disc. 

 From the stomach five branches pass off, one to each ray. Each 

 branch divides into two caeca, which lie one on either side of the 

 ray. From the stomach a short rectum leads upward to the anus, 

 which opens on the abactinal surface at a little distance from the 

 centre. Two small longitudinally folded diverticula from the 

 rectum occur below the anus. These rectal caeca occupy a 

 similar position to the " respiratory trees " of Holothurians, with 

 which they may be homologous. 



The Water- Vascular System consists of a circular vessel round 

 the oesophagus (the circumoesophageal canal or water- vascular ring), 

 from which, in a five-rayed starfish, there are eleven offsets. The 

 most important are the five radial canals, one of which passes 

 along each ray, just external to the ambulacral ossicles. From 

 these radial canals branches are given off on either side ; each 

 branch ends in a tubular podion, which consists of an internal 

 reservoir or ampulla situated above the ambulacral ossicles, and of 

 an external tube or sucker. Valves occur on the transverse branches, 

 and prevent water, expelled from the ampulla, returning into 

 the radial vessel ; they thus direct it into the sucker. The next 

 set of offsets from the circumoesophageal canal are five sac-like 

 " Polian vesicles," one in each interradius ; they act as reservoirs 

 for the water-vascular system. The last (eleventh) vessel on the 

 circumoesophageal canal is the " stone-canal/' which runs from 

 the base of one of the Polian vesicles to the upper surface of the 

 starfish. It expands above, and its upper end is attached to the 

 madreporite, through the pores in which water enters the water- 

 vascular system. 



The circumoesophageal canal also supports nine tufts of 

 tubules known as Tiedemann's bodies ; there is a pair of tufts 

 in each interradius, one on either side of the base of the " Polian 

 vesicle," but in the interradius containing the stone-canal there is 

 only a single Tiedemann's body. 



The presence of a blood-vascular system in Asteroids is not yet 

 determined, the organs described as such belonging to the Pseud- 

 haemal System (cf. pp. 22, 26). The main organ in this system 

 is the "axial sinus," which is a rather thick vertical tube sur- 

 rounding -the stone-canal. It communicates below with a ring (the 

 circumoesophageal pseudhaemal ring), which surrounds the mouth 

 and gives off five radial branches, which pass one along the upper 

 side of each ray. At its upper end the axial sinus communicates 

 with the genital ring ; attached to this ring are five pairs of short 

 processes, while an additional pair passes beside a prolongation of 

 the axial sinus leading to the madreporite. Some of the pores of 

 the madreporite open to the axial sinus, and there is no known 

 direct communication between the latter and the stone-canal. 



