168 



COMPAEATIVE ANATOMY. 



one anotlier. New longitudinal trunks are formed from tlie vessels 

 distributed to the enteron. While in these forms a complex system 

 is produced by the combination of the primitive median trunks with 

 a system of canals formed from lacunae of the coelom, the whole 

 vascular system may be made simpler by the disappearance of these 

 median trunks. This is the case in Nephelis, where there is a wide 

 median sinus^ and two lateral vessels. 



This form of vascular apparatus^ developed 

 out of a lacunar system, is limited to the Hiru- 

 dinea, for in the Annelides the vascular system 

 is almost always shut off from the coelom. Where 

 it is not so, we have to do, not with further 

 development, as is the case in the differentia- 

 tion of the coelom of the Hirudinea, but with 

 degeneration. 



The dorsal vessel lies, as a rule, immediately 

 upon the enteric canal, and often appears to be 

 embedded in the same glandular layer. In addi- 

 tion to anterior and posterior connecting vessels, 

 there are lateral vessels which correspond with 

 the metameres. They are divided into those which 

 directly surround the enteron, and form a capil- 

 lary network, often a highly-ramified one, in its 

 walls (visceral vessels), and into those which pro- 

 ject into the coelom, and run either along its 

 walls or its appendages (parietal vessels). In the 

 Scoleina the arrangement is generally the same 

 throughout the whole body. In many cases the 

 transverse vessels, as well as the dorsal longitu- 

 dinal trunks, are pulsatile, and one or more pairs 

 are considerably widened (Fig. 77, c). In this 

 differentiation of a portion of the vascular system 

 we see the beginning of the development of a 

 central organ of the circulation — a heart. The 

 ventral vessel is very seldom contractile. Fresh 

 complications in structure are due to the develop- 

 ment of fine vascular networks, such as, in Lum- 

 bricus for example, are distributed as capillaries 

 through the body. Branchiobdella is allied in 

 the characters of its vascular system to the simpler conditions found 

 in the Scoleina. 



§ 139. 



The development of the respiratory organs is of influence in 

 producing changes in the distribution and differentiation of the 

 blood vascular system. In the Scoleina there are no distinct organs 

 of this kind, and either the whole surface of the body, or, by the 

 introduction of water into it, the coelom, acquires a respiratory 

 function. Consequently we do not see any great variations of the 



Anterior 

 portion of the blood 

 vascular system of 

 Ssenuris varie- 

 g a t a. d Dorsal ves- 

 sel. V Ventral vessel. 

 c Heart-like enlarge- 

 ment of a transverse 

 anastomosis. The 

 arrows indicate the 

 direction of the cur- 

 rent of blood. 



