THE CRINOIDEA 103 



believe that the water-ring opened into the body-cavity by a single 

 ciliated canal in the posterior interradius ; and that the body-cavity 

 communicated with the exterior by a single hydropore in post. A, 

 sometimes merged in the anal opening (as probably in Blastoidea). 

 This system was not as yet completely affected by radiate sym- 

 metry ; but in some forms it became so by the development of a 

 similar canal with corresponding hydro- 

 pore in each of the other interradii (Fig. 

 X., compare Figs. XXXIV., XLVL, and 

 CXIV.). 



The two subtentacular canals of each 

 arm enter a division of the coelom that 

 passes down the vertical axis through 

 the coil of the gut, and is known as the Fl0t X- 



"axial sinus." The dorsal coeliac canal 

 passes into a division of the coelom that 



SUrrOUnds both axial sinUS and ffUt, and connective tissue fibres ; g, wall of 



11 j ,-, ,, ,., i , gut; n, circumoesophageal nerve ; 



IS Called the pen - intestinal Cavity. oe, oesophagus; p, pore; r.c, ring 



The remainder of the coelom, surround- ^.su, stone canal ; r, stereom 

 ing the latter, is called the " subtegu- 



mentary cavity." All these divisions of the body -cavity are 

 lined by endothelium, and are separated from each other, as 

 well as penetrated, by connective tissue, in which spicules are 

 often richly developed. From the peri -intestinal cavity, at its 

 aboral end, there are in this way cut off five chambers, which 

 surround the axial sinus, and are themselves covered on all sides 

 by epithelium, containing ganglion -cells and nerve-fibres ; the 

 whole structure is called " the chambered organ " (see Fig. XX. 

 p. 24). 



The genital rachis of each arm is connected with a complex of 

 twisted, fine canals, called the "axial organ" (see p. 23). This 

 passes down the axial sinus, widening in the middle of its course, 

 and then narrowing to a thin strand as it passes between the 

 five chambers just mentioned. 



The axial nerve-cord of the arm does not, as all the organs yet 

 dealt with, pass to the oral centre, but enters the v theca over 

 the radial. If there is a separate axial canal, it may be continued 

 through the radial facet into the substance of the thecal plates. 

 The cords ultimately pass into the epithelial covering of the 

 chambered organ, but their passage is not a direct one (Figs. XI. 

 and XII. ). Each cord is really a double structure, connected 

 at intervals by chiasmas, and so soon as it enters the radial it 

 divides into two branches, one of which proceeds to the basal on 

 the right, the other to that on the left. In addition the branches 

 are connected with each other and with those of the other radii by 

 a series of commissures that form rings all round the cup. One 



