XXIV BLOOD-VESSELS 279 



body. In Hydra, where the lining wall of the digestive 

 cavity is in direct contact with the simple wall of the body 

 the products of digestion can pass at once by diffusion from 

 endoderm to ectoderm, but in the present case a means of 

 communication is wanted between the enteric epithelium 

 and the comparatively complex and distant body-wall. The 

 peptones and other products of digestion diffuse through 

 the enteric epithelium into the ccelomic fluid, and by the con- 

 tinual movement of the latter — due to the contractions of 

 the body-wall — are distributed to all parts. Thus the 

 external epithelium and the muscles, as well as the nervous 

 system and reproductive organs, not yet described, are 

 wholly dependent upon the enteric epithelium for their 

 supply of nutriment. 



AVe have now to deal with structures which we find for the 

 first time in Polygordius, namely blood-vessels. Lying in 

 the thickness of the dorsal mesentery is a delicate tube (Fig. 

 67, A and c, D.V.) passing along almost the whole length of 

 the body r this is the dorsal vessel. A similar ventral vessel 

 (V.V) is contained in the ventral mesentery,^ and the two are 

 placed in communication with one another in every segment 

 by a pair of commissural vessels (a, Cotn.v) which spring right 

 and left from the dorsal trunk, pass downwards in or close 

 behind the corresponding septum, following the contour of 

 body-wall, and finally open into the ventral vessel. Each 

 commissural vessel, at about the middle of its length, gives 

 off a recurrent vessel (R.V.) which passes backwards and 



^ The statement that the dorsal and ventral vessels lie in the thickness 

 of the mesenteries requires qualification. As a matter of fact, these 

 vessels are simply spaces formed by the divergence of the two layers of 

 epithelium composing the mesentery (Fig. 67, C, and Fig. 70, A) : only 

 their anterior ends have proper walls. 



