170 



COMPARATIVE ANATOMY. 



vascular ramifications. A breaking-up such, as affects the parietal 

 transverse anastomoses in the gills, may affect also the longitudinal 



trunks, which then take 

 the form of a continuous 

 network of vessels, from 

 which new passages are 

 developed. The pheno- 

 mena, which give rise to 

 a collateral circulation, 

 must also furnish us with 

 the explanation of these 

 relations. Thus in Poly- 

 ophthalmus the median 

 trunk is broken up along 

 the course of the mid-gut. 

 Two dorsal and two ven- 

 tral trunks arise from 

 the anterior and posterior 

 simple median vessels 

 in the Hermellidas ; in 

 Eunice the ventral, and 

 in Nephthys the dorsal, 

 pair are present. 



In some the vascular 

 system is atrophied (Gly- 

 cera, Oapitella). 



A combination of the 

 types of the vascular sys- 

 tem, which obtain in the 

 Annelides and the Ne- 

 mertines, can be made 

 out in Balanoglossus. 

 This consists in the pre- 

 sence of both median 

 and lateral longitudinal 

 trunks, but their visceral 

 branches partly function 

 as branchial vessels, and 

 so give rise to an arrangement which is very different from that of 

 the majority of Vermes. 



§ 140. 



The vascular system of the Gephyrea presents characters which, 

 not only in their relations to the circulatory system of other Vermes, 

 but even when we compare their various conditions with one 

 another, are by no means easy to understand ; at the same time 

 there are many important lacunas in our knowledge of the ana- 

 tomical facts. The most important of these is the question whether 

 there is any connection between the cavities of the vascular system 



Fig. 79. Vascular system of T ere bell a nebu- 

 losa ; opened from the dorsal surface, t Tentacles 

 (not fully drawn), br Three pairs of branchiae 

 ph Muscular portion of the fore-gut (pharynx). 

 v Enteron. vd Dorsal vessel, vv Ventral vessel 

 (after Milne-Edwards). 



