584 COMPARATIVE ANATOMY CHAP. 



sarily also have slanting systems of blood vessels, which open into the anterior 

 end of the ventral vessel of the trunk. 



In Ptychodera, on the contrary, where the vascular folds descend perpendicularly 

 in the anterior part of the collar region, and form a ring around the buccal cavity, 

 which is then continued as a medioventral fold to the posterior end of the collar, 

 the vessels which run in these folds naturally also form a similar ring, which 

 passes into a medioventral collar vessel continuous with the ventral vessel of the 

 trunk. 



The vessels of the collar are distinguished from the principal vessels of the 

 trunk by the fact that they possess no musculature. 



A circular space, running in the limiting membrane of the septum separating 

 the 'collar from the trunk, is in open communication with the ventral vascular 

 trunk. 



6. The vascular capillary networks of the integument and of the intestine are 

 everywhere in communication with the two principal vessels. In the collar, a 

 connection is formed between the integumental and the intestinal plexuses by 

 plexuses running in the mesenteries. Where peripharyngeal cavities are found 

 (Ptychodera, Schizocardium} the integumental plexus lies in the peripheral walls of 

 the cavities, viz. those turned to the ccelom of the collar. Of all the sections of 

 the intestine, the hepatic is most distinguished by the closeness of its capillary 

 plexus and its rich supply of blood. 



In some species of Ptychodera, dendriform, blindly-ending, vascular cseca project 

 from the dorsal side of the collar cord and sometimes also from the dorsal septum. 



7. Lateral vessels Ptychodera. Two lateral vessels, provided with muscular 

 walls, run through the branchiogenital and hepatic regions. They originate 

 anteriorly out of the vascular network of the integument, run backward in the 

 submedian line, and enter the lateral mesenteries, in whose limiting membranes 

 they run. At the posterior ends of these mesenteries, at the boundary between 

 the branchiogenital and the hepatic regions, they pass over on to the intestine, 

 being continued in two vessels running along close below the liver-caeca of the 

 intestine, and finally open into the intestinal capillary network. The anterior 

 portions of the lateral vessels, which might be called the genital vessels, are 

 connected with the capillary network of the gonadial walls. Similar lateral 

 vessels also occur in Schizocardium. 



In Balanoylossus and Glandiceps there are, usually in the hepatic region, two 

 lateral vascular trunks of the intestine, which open anteriorly and posteriorly into 

 its capillary network. Their musculature consists, in Glandiceps, of circular 

 fibres ; in Balanoglossus, of longitudinal fibres. These perhaps correspond with 

 the posterior or intestinal portion of the lateral vessels of Ptychodera. 



8. The branchial vessels. These vessels have been best investigated in Ptycho- 

 dera. A branchial capillary network is found in the limiting membranes both in 

 the branchial tongues and the branchial septa, i.e. in the limiting membrane 

 which separates the epithelium of the branchial intestine from the visceral layer of 

 the mesoderm which lies outside it. Into this plexus, vessels having a definite con- 

 stant course and of large size enter : (1) a vessel along the back of each tongue, (2) 

 a vessel along the inner side of each lingual prong, i.e. on the side turned to the 

 lingual cavity, (3) a vessel along that edge of each septal prong which is turned to 

 the body wall. These last-named vessels run ventrally into the capillary network 

 of the lower, nutritive part of the branchial intestine (i.e. of the oesophagus), 

 and must be considered as efferent vessels of the branchial septa. 



The branchial capillary network receives its blood from afferent branchial 

 vessels, which originate out of the dorsal vessel and (in Ptychodera clavigera] have 

 the following arrangement : Each afferent branchial vessel, soon after its origin 



