/ / 



are absent. It is not improbable, however, that it functions as a reservoir of blood derived 

 from the walls of the stomach. The movement of the blood in the vessel mieht well be effected 

 by the pressure of the coelomic fluid induced by contractions of the body-wall. 



There can thus be no question that the dorsal vessel is an important structure in 

 Cephalodiscus. Examining its ventral end (figs. 33, 37) it is found to end abruptly on the 

 surface of the dorsal caecum of the stomach. In most cases I cannot certainly demonstrate 

 any vessels entering it from the stomach; but the evidence of the bud shewn in PI. V, figs. 

 57, 58 is in favour of the view that it receives its blood from a system of sinuses lying 

 between the gastric epithelium and the limiting membrane of the body-cavity. 



Vessels of the gonads. 



Near its dorsal end, the dorsal mesentery gives off a pair of lateral mesenteries (figs. 

 120 — 123, ov. III.) which are inserted into the entire length of the oviducts in the female, and 

 into the dorsal part of the internal walls of the ovaries. Each lateral mesentery carries a 

 conspicuous vessel (PI. XII, fig. 151 ; PI. V, fig. 45, ov.v.) which passes to the wall of the 

 ovary, somewhat ventrally to the internal end of its pigmented duct. The large amount of yolk 

 deposited in the eggs of Cephalodiscus implies an abundant nutriment supplied to the ovaries-, 

 and I think it may thus fairly be inferred that the blood passes from the wall of the stomach 

 in a dorsal direction to supply the ovaries. In the male C. sibogae, a similar arrangement 

 obtains. The dorsal vessel (PI. VHI, figs. 87, 86) extends from the dorsal surface of the bend 

 of the vestigial alimentary canal to the dorsal ends of the testes, which are supplied by large 

 vessels passing along paths which may be regarded as lateral mesenteries homologous with 

 those of the ovaries. The vestigial character of the alimentary canal in the male offers, however, 

 some difficulty as to the source of the nutritive fluid contained in the vessel. This question 

 will be considered under the heading of the male C. sibogae (p. 88). 



Vessels of the stalk. 



These vessels occur in the position already described by Masterman, on what he terms 

 the "ventral" and "dorsal" sides of the stalk respectively. In view of the orientation adopted 

 in this Report, according to which the entire stalk is a ventral appendage of the body, I 

 prefer to call the vessels respectively "anterior" and "posterior", and to regard them as two 

 limbs of a loop-like ventral vessel which extends into the stalk. The continuity of the two 

 vessels at the basal end of the stalk has not however, been demonstrated with certainty. 



The vessels are seen in transverse section, in C. dodecalophus (PI. XI, fig. 133), as 

 small dilatations of the two ridges of basement-membrane which alone represent the ventral 

 mesentery of the stalk. It may be supposed that primitively the ventral mesentery extended as 

 a complete partition down the stalk. With the filling up of its cavity which this organ has 

 for the most part undergone by the development of the longitudinal muscles and their investing 

 connective tissue, the ventral mesentery has persisted as a distinct structure at the anterior 

 and posterior sides only. 



The condition of the stalk-vcsscls is probably not cjuite sim])lc at the basal e.xtremity 



