76 



to me correct for the "ventral" or anterior vessel; but a moments consideration will shew that 

 it would be impossible for the median "dorsal" vessel of the stalk to be a direct continuation 

 of the dorsal vessel of the bod)- unless it could traverse, or pass to one side of, the second 

 stomach and intestine. Xor can I agree with the statement that each tentacle has a vessel on 

 the dorsal side of its coelomic cavity. It is easy to find appearances similar to those shewn 

 by M.-vsTERMAN (97, 2, Plate XXVI, fig. 27). With certain methods of staining the crescentic 

 area seen in the transverse section appears to be hollow. But in other cases it seems to be 

 clear that the crescentic appearance is simply due to a thickening of the basement- membrane 

 lining the coelomic cavity; and I believe that Masterman's tentacle-vessels are in fact merely 

 skeletal thickenings of that membrane. 



My own results on the "vascular system" of Cephalodiscus may be considered under two 

 heads: — (i) those spaces whose vascular nature is practically certain, and (2) those with 

 regard to which some caution in interpretation seems to be desirable. 



(1) Under the heading of spaces the vascular nature of which is practically certain, 1 

 include the vessel which lies in the dorsal mesentery of the metasome, adjacent to the wall 

 of the pharynx: the vessels given off from this to the reproductive organs; the two ve.ssels of 

 the stalk, and their continuaticjiis in the body; and lastly the "heart", or pericardial sinus, 

 which is invaginated into the posterior wall of the pericardium. .All these parts have been 

 described by Masterman in one or other of his accounts. Under the heading (2) of spaces 

 with regard to whose? interpretation as vessels it is well to e.xercise some caution I include 

 practically all the other "vessels" described by M.vstermax. 



Dorsal vessel. 



This name may be employed for the vessel which is contained in the dorsal mesentery 

 of the metasome. Evidence of its existence can be obtained in most specimens of Cephalodiscus. 

 In those in which the vessel has been jireserved in a dilated condition it forms an extremely 

 conspicuous organ, and it is in fact far the largest vessel in the animal. 



The dorsal vessel {d.v}j is shewn in the combined .sagittal .section of C. gracilis (PI. 1\', 

 fig- 37). '■! the actual sagittal sections (PI. V, figs. 43 — 46) and in the frontal sections (PI. VI, 

 figs. 67 — 70) of the same species, i'l. Ill, fig. 22, reconstructed from the series of sagittal 

 sections by the ground glass method, shews this vessel as a large cylindrical space which starts 

 irom the anterior caecal projection of the stomach, and then pas.ses in an obliquely dorsal 

 direction in such a way as to lie, in the whole of its course, parallel to the dorsal wall of the 

 pharynx. The vessel is also conspicuous in the sagittal section (fig. 33) of C. levinseni, where 

 it has the same relations. It is .shewn in transverse section (PI. XII, figs. 152—157) in C. 

 dodecalophus, and lastly it is clearly seen in both neuter (PI. \'11I, figs. 90, 91) and male 

 individuals (figs. 86 — 88) of C. sibogac. The vessel in question is particularly clear in the male 

 individual represented in figs. 86 — 88, and in this case, as 1 have also observed in the neuter 

 of the same species, it contains an obvious coaguluni. Tiie walls of the vessel are usually very 

 thin, although in C. Icvinsc7ti (fig. 33) it appears to be fairly thick. I have not been able to 

 demonstrate any system of muscles connected with its wall, allliough 1 am not sure that the\- 



