312 A. T. MASTERMAN. 



cavity. In every case iu which a true vessel is present it is 

 formed by the thickened mesoblastic walls surrounding the 

 vessels. 



The vascular system is, therefore, morphologically in a very 

 primitive condition. 



There is just under the nerve-ganglion a large sinus caused 

 by a want of contiguity between the mesoblastic walls of the 

 pre-oral cavity in front and the collar cavities behind. This 

 sinus, which may be termed the subneural sinus {s. n. s.), is 

 seen in figs. 1^ 3, 20, 23, and 38, in which its relationships can 

 be readily made out. Into its cavity projects upwards from 

 below the inner end of the subneural gland, so that the 

 lining cells of this organ are in direct contact with the hsemal 

 fluid. At the posterior extremity of the sinus it leads by a 

 chink or fissure between the gut wall and that of the two 

 collar cavities, along the dorsal wall of the oesophagus (figs. 7, 

 8, d. b.v.) to the hind extremity of the collar region. Here it 

 falls into the large dorsal vessel with contractile walls. This 

 vessel is continued along the dorsal wall of the pharynx and 

 stomach (PI. 19, figs. 16 and 17, d. h. v.) till it meets the intes- 

 tine. Here a small ring-sinus is formed connecting it with the 

 ventral vessel with csecal prolongations into the body cavity. 

 At the anterior extremity of the dorsal vessel, at the front 

 end of the pharynx, two lateral trunks are given off, which, 

 passing downwards round the base of the oesophagus and 

 between this and the notochords, meet again in the mid-ventral 

 line, forming a post-oral ring sinus (PI. 20, fig. 21, r. s.). From 

 the point of junction (fig. 20, v. b. v.) there originates a mid- 

 ventral vessel which may be traced throughout the length of 

 the gut (figs. 16 and 17, v. b.v.) into a large haemal ring 

 immediately internal to the perianal ciliated band. Lastly, 

 there are some not very distinct indications of haemal sinuses 

 passing down the tentacles ; but these are not very decided. 



This blood system is closely comparable to that of Balano- 

 glossus and of Cephalodiscus, as I hope to show later. 



Caldwell makes the statement, " Free communication exists 

 between the body cavity in front of the septum and the split 



