-8 



of the stalk. It is complicated by the development of the buds, which, as Masterman has 

 shewn (98, 2, p. 515) receive branches from one of tlic stalk-vessels. There is some evidence 

 in C. sibogac that the two vessels communicate b\- means of a plexus at the basal end of the 

 stalk ; but whether the connexion is usually of this nature, or whether, as some sections seem 

 to indicate, the two vessels form a simple loop at the base of the stalk, it is probable that 

 they may be regarded as respectively afferent and efferent in function. 



The anterior vessel passes from the stalk along the anterior part of the ventral mesenterj- 

 of the body (figs. 29, 126, 125, a.v.^a.v))^ where it lies close to the body-wall; and in suitable 

 specimens it can be distinctly traced as far as septum '/.■• The posterior vessel, on entering the 

 body, passes in the ventral mesentery as far as the wall of the alimentary canal. In C. gracilis 

 (PI. VI, fig. 71, PI. \', fig. 53, and the reconstruction I'l. HI, ^•g. 22) it ends on the surface 

 of the second stomach as a distinct dilatation, the structure of which cannot clearly be made 

 out, although in one specimen the dilatation has been observed to receive muscle-fibres from 

 the longitudinal layer of the anterior wall of the metasome. The ending of the posterior stalk- 

 vessel on the second stomach is also somewhat dilated in C. dodecalopJms (PI. XIII, fig. 169). 



The clearest proof that the structures here described as vessels are really of that nature 

 is obtained from C. gracilis. The stalk-vessels are here unusually large (figs. 28, 29, 134, 135); 

 and although they are commonly displaced so as to take u]) a lateral po-sition, a careful 

 examination of the stalk usually serves to shew that they are connected respectively with its 

 anterior and posterior walls. The connexion is, however, far less obvious than in C. dodecalophus 

 and C. Icvitiseni, since remains of the ventral mesentery connecting the vessels with the body- 

 wall are not easily discovered. The vessels are large, thin-walled cavities lying internally to the 

 muscular layer, in the sparse connective tissue which occupies the central part of the stalk in 

 this species. 



The stalk-vessels of C. gracilis are sometimes very conspicuous in entire preparations of 

 young buds. They are shewn in PI. Ill, figs. 28, 29; and in this sijecimen it could be proved 

 that the posterior vessel becomes attached to the wall of the alimentary canal of the bud, while 

 the anterior vessel passes along the inner side of the median line of the anterior wall ol the body. 



Attention may here be called to the account which has recently been given by Fowler 

 (04, ]). 23) of the structure of Rliabdopleura. The resemblance between the stalk, as described 

 by Fowler, and the stalk of Cephalodiscus is practically complete, both as regards mode of 

 origin and minute structure. Fowler's fig. 2 might stand for the stalk of C. Icvinscni almost as 

 well as for that of Rhabdopletira, the completeness of the mesentery and the difference in size 

 e.Kcepted. It shews a triangular ventral or anterior thickening of the epidermis which has "very 

 much the appearance of a .superficial nerve". The longitudinal muscles are related to those of 

 the body exactly as in Cephalodiscjis. The cavity of the stalk is divided by a median mesentery, 

 in which run two canals. The one next the nerve [b) was not found in all specimens, Init the 

 one {end}) which occurs at the middle of iIk- mesentery is always present and is "generally 

 completely filled with a granular mass". 



1 think there can be no doubt that these two cavities are the homologues of what 

 M.vsterm.w has described as the stalk-vessels of Cephalodiscus^ and indeed Fowler admits that 



