THE ENTEROPNEUSTA. 58 



(3) in B. minutus a pair of large lateral vessels {v. fig. 93) 

 in the digestive region. These are connected by plexuses in 

 the skin and under the epithelium of the gut. In the operculum 

 this capillary system of the skin forms a more or less definite 

 circular vessel. In parts of their course these vessels are always 

 more or less filled with a fibrous-looking substance, apparently 

 cellular, which lines the walls (fig. 71). The generative 

 organs lie in blood-sinuses derived from the subcutaneous 

 plexus. 



I stated (Part I) that the branchial blood-supply resembled 

 that of Amphioxus. From further observation I have come 

 to the conclusion that this is a mistake, and that the 

 vessels supplying the gills are all derived from the dorsal 

 vessel, as Spengel has stated, being, in fact, merely the skin 

 capillaries of the dorso-lateral regions. The main vascular 

 trunks are all formed from the mesoblast of the first cavity 

 and of the third pair of cavities. The capillaries under the 

 skin and round the gut are formed in situ in the mesoblastic 

 walls in which they occur. 



The Generative Organs. 



The Ovaries. — The animals are all dioecious. The origin 

 of the ovaries is not certain, but there is very strong evidence 

 that they are epiblastic. At all events, from almost their 

 earliest appearance, they are connected with the skin in the 

 dorso-lateral regions {ov. fig. 110). It is almost impossible to 

 believe that an attachment of this kind is secondary, and I 

 have never seen an ovarian follicle entirely separate in the 

 body cavity. 



Soon after its appearance it consists of a mass of loose round 

 cells. A cavity next appears in its interior, as though due to 

 a disintegration, and after the appearance of this cavity the 

 cells bounding it develope into ova (figs. Ill and 112). 



The egg-shell appears soon as a close-fitting membrane. The 

 germinal spot is enclosed in a remarkably tough membrane in 

 all the species examined. Though the ovaries are connected 



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