570 THE INVERTEBRATA 



course under the shell by the radial nerve cord, epineural canal, and 

 perihaemal canal (Fig. 428). It is said that a small radial "blood 

 vessel" (not shown) runs between the perihaemal canal and water 

 vessel. From the water vascular ring the stone canal, which is not 

 calcified, ascends vertically to the madreporite, accompanying the 

 axial organ, which surrounds the small axial sinus. Under the 

 madreporite, however, the sinus is free and enlarged and forms an 

 "ampulla" into which the stone canal opens. 



Fig. 428. A diagram of a section across a radius of Echinus. From Shipley 

 and MacBride. amb. ambulacral plate; amp. ampulla; bo. boss for articula- 

 tion of spine; ect. ectoderm; epin. epineural canal; m. muscles which move 

 the spine; ra.n. radial nerve cord; ra.peh. radial perihaemal canal; ra.wv. 

 radial water canal; sp. spine; t.f. cavity of tube foot. 



The oral ring of the lacunar system lies below the water vascular 

 ring. The features of this system have been described on p. 553. The 

 gonads are five large masses hanging into the perivisceral cavity from 

 the region of the genital plates. The rachis is degenerate in the adult. 



Echinus is an example of the regular sea urchins (order Endocyclica). 

 The class contains two other orders, Clypeastroida and Spatangoida, 

 known collectively as the irregular urchins (Exocyclica), in which the 

 anus, with its periproct, is displaced from the apical position which it 

 occupies in the regular forms, and lies in an interradius, known as the 

 posterior interradius, so that the body, which is considerably or very 

 much flattened, has a marked bilateral symmetry. The madreporite 



