VASCULAR SYSTEM OF VEEMES. 



169 



Fig. 78. Diagrammatic transverse section through 

 the hinder half of the body of Arenicola, to show 

 the arrangement of the vessels. D Dorsal ; Y 

 Ventral side, n Ventral medulla. i Enteric 

 cavity, hr Branchiae, v Ventral vascular trunk. 

 ah Branchial vessels, d Dorsal vascular trunk. 

 ]\ Branch surrounding the enteric canal, v ' Visceral 

 ventral vessel. 



vascular system in various portions of tlie body^ and it is only in 

 some wliicli live in tlie mud of fresh water, and in wliicli the hinder 

 part of the body takes a special share in respiration, that the parietal 

 vascular coils present any very great develo^^ment (Lumbriculus) . 



Among the Chfetopoda also these simpler relations obtain, but 

 the greater differentiation 

 of the head, and of the 

 fore-gut, is followed by 

 some changes in the 

 vascular system. When 

 branchiaa are present the 

 parietal vascular system 

 is continued into them ; 

 in the simplest case a loop 

 of the vessel passes into 

 the appendage, which has 

 the function of a gill. 

 Here we have the com- 

 mencement of the gradual 

 separation of an arterial 

 and a venous portion. 

 This arrangement is re- 

 peated when the branchiae are distributed over a large number of 

 metameres, as in Eunice and Ai-enicola. The dorsal trunk gives 

 off, in addition to the vessels for the enteron, others which pass 

 to the laterally -placed branchite ; from each of these a vessel passes 

 to the ventral trunk (Fig. 78). The same characters obtain in the 

 Hermellidte, where the branchiae have but a single central cavity, 

 and where, therefore, there can be no anatomical separation of the 

 efferent and afferent blood. In Arenicola these characters obtain in 

 the hinder half only of the body. In the anterior half one branchial 

 vessel passes to the chief ventral trunk, and the other to a visceral 

 ventral vessel. 



When the respiratory appendages are limited to a smaller portion 

 of the body, as is the case in the Tubicola;, there is a corresponding 

 increase in the difference between the development of various 

 vascular regions. Thus in the Terebellidte (Fig. 79) the dorsal 

 vessel ivd), above the muscular pharynx, is widened out into a large 

 tube, which supplies the branchiae (6?-), and so functions as a 

 "branchial heart." Afferent vessels pass from the branchise to 

 the ventral vessel. In many the function of a central organ is taken 

 on by transverse anastomoses (Scoleina). A vessel of this kind, 

 which passes from the ventral to the dorsal vessel, is found even in 

 the Terebellidfe, where it functions as a part of the cardiac division 

 of the dorsal vessel. In Az'enicola this vessel is continuous with two 

 very wide transverse vessels, which pass to the ventral trunk. 



The arrangement of blood-vessels is constant enough when they 

 are not richly distributed ; but this is not the case in those divisions 

 in which the enteric-wall and the body- wall are largely provided with 



