THE AKATOMT OP CAUDINA COHIACEA. 



461 



•nerve-cord with its associated coelomic canals *. The nerve-cord 

 is as usual divided into inner and outer bands, separated from 

 one another by a thin layer o£ connective tissue. In the inner 

 band (fig. 7, i.h.) the nerve-cells are aggregated on the inner aspect, 

 and in the outer band (fig. 7, o.h.) they are aggregated on the 

 outer. Between the inner band and the blood-sinus lies the 

 hypoueural t coelomic canal (figs. 7 & 9, hyp.). Outside the outer 

 .band lies the epineurat coelomic canal (ep.). 



If we trace these various radially-placed organs to the hinder 

 extremity of the body, we find that they terminate respectively 

 as follows : — The longitudinal muscle-band gradually diminishes 

 in thickness, the fissure between its two halves disappears (fig. 8) 

 and the muscle then dies out (figs. 5, 9, 10, 11), its place being 

 taken by bands of transverse fibres (figs. 5, 10, 11, t.m.) placed 

 in the radii around the anus, and continuous with the inter- 

 radially-placed transverse or circular muscle-bands. Just posterior 

 to each radially-placed transverse muscle-band the integument 

 projects to form a supposed vestigial anal tooth, which consists 

 of a solid mass of connective tissue containing spicules and 

 surrounded by the epidermis (figs. 1, 6, 12, t.). 



The radial ambulacral vessel, after the termination of the 

 longitudinal muscle, increases greatly in diameter (fig. 9, r.a.) 

 ^nd gives off two or three wide branches on each side (figs. 5, 10, 

 11, amp.). The main trunk and its branches terminate respec- 

 tively in the central and lateral anal tentacles of each group 

 (fig. 12). Just before it gives oflT its lateral branches, the radial 

 ambulacral vessel appears to lose the layer of muscle-fibies from 

 its outer aspect (fig. 9), but these appear again after the division 

 has taken place, as a very distinct layer of longitudinal fibres on 

 the outer aspect of each branch (fig. 11, in./.). At first the 

 swollen branches of the ambulacral vessel appear to partake 

 somewhat of the nature of ampullae, and they probably function 

 as such in controlling the protraction and retraction of the anal 

 tentacles, by forcing liquid into them or causing it to be with- 

 drawn. It will be seen from a comparison of figs. 11 and 12, 

 that in the specimen from which the sections were taken, these 



* It. seems pretty safe to assume that these cauals are coelomic, although. I 

 believe that their derivation from the body-cavity has not been definitely 

 .established. 



t " Pseudohsemal canal " of some authors, " hyponeural canal " of Gerould. 



