ECHINOIDEA. 231 



The perihaemal system consists of (1) five radial canals, extend- 

 ing the whole length of the radii and lying between the radial nerve 

 and the radial water- vessel (Fig. 169, 18), and (2) a large sinus 

 round the mouth. This is the perioesophageal sinus or lantern- 

 coelom, so called because, in the forms with teeth, the jaws 

 (lantern of Aristotle) lie within it. It is homologous with the 

 outer perihaemal ring of other classes, though it is completely 

 separate in the adult from the radial perihaemal vessels which 

 reach right up to it. 



The external gills, of which there are five pairs, one pair in 

 each interradius, are processes of the outer part of the peri- 

 stomial membrane and contain prolongations of this sinus. They 

 pass through notches on the interambulacral marginal plates 

 and are present in most Endocyclica, but are absent in the 

 Cidaroida. 



The internal gills or Stewart's organs are found in Cidaroida 

 and Echinothuridae. They are hollow processes of the lantern 

 membrane into the body cavity and their cavity is a prolonga- 

 tion of the perioesophageal sinus. They are usually five in 

 number and radial in position. Organs of a similar nature, 

 eight or nine in number, have been described by Cuenot in some 

 Clypeastroids. 



The axial sinus ends blindly, ventrally, at some distance from 

 the oral region, while dorsally it communicates with the stone- 

 canal. It is contained in the axial organ, which is wrapped 

 round it. There is no inner circumoral perihaemal ring. 



There is also an aboral circular sinus in the walls of which 

 lies the generative rachis (p. 234). This sinus sends prolonga- 

 tions to the generative organs. 



The water- vascular system is arranged very much as in Asteroids. 

 There is a circumoral vessel (Fig. 171, Eg) placed at the upper 

 end of the pyramids of the jaws and giving off the five radial 

 canals which, passing beneath the intermediate plates, travel 

 oralwards within the lantern-membrane to the test and then, 

 after giving off a branch to the oral tube-feet (absent in Cidaroida 

 and Echinothuridae) which perforate the plates in the peristomial 

 membrane, turn outwards through the auricles to run along 

 the radii within the shell plates to terminate in the short un- 

 paired tentacle which perforates the ocular plate. In most 

 Endocyclica the circumoral vessel possesses in each interradius 



