76 



OLIGOCHAETA 



Fig. 20. 



the exterior in the Gymnophionae. The physiological meaning of ' intra-epithelial blood- 

 capillaries,' as Lankester has termed these structures in the leech, whei-e he was the 

 first to discover their existence, seems to be clear. One of the characteristics of the 

 Oligochaeta is the rare occurrence of branchiae ; Dero, Chaetobranchus, Branchiura, 

 Hesperodrilus branchiatus and the questionable Alvna are the only real (or reputed) 

 Oligochaeta which form branchiae. The absence of these special organs is compensated 

 for by the utilization of the entire skin as a branchial organ. The efiiciency of the 

 skin as a branchial organ must be largely increased by the penetration of the 

 capillaries into the outermost layers. 



Bourne was led by his researches into the vascular system of Megascolex coeruleus 



to a reasonable view of the course of the blood 

 in that species which will probably, as he thinks, 

 prove to be more widely characteristic. 



Excepting at the head end of the body, where 

 the vascular system like other organs shows 

 a cephaUzation, it is metamerically arranged. 

 Moreover the fact that an earthworm . can live 

 after a considerable part of its body has been 

 shorn away indicates that each segment is to some 

 extent independent in its circulatory mechanism. 



There is no doubt — all observers agree upon 

 that point, which can be easily seen in the living 

 worm— that the blood flows forwards in the 

 dorsal vessel. In the cephalized region of the 

 body the blood passes down into the hearts ; 

 these contract from above downwards, just as 

 the dorsal vessel contracts from behind forwards. 

 In the intestinal region of the worm there are two sets of vessels connected with 

 the dorsal vessel— the dorso-integumentary and the dorso-intestinal. It appears that 

 blood flows from the latter into the dorsal vessel, and out of the dorsal vessel into 

 the former. This view is not that universally accepted. In the anterior region of 

 the body of course, the supra-intestinal vessel is supplied from the same source as 

 is the dorsal vessel posteriorly. The ventral vessel of Megascolex communicates with 

 the dorsal vessel anteriorly and directly by the hearts ; posteriorly it communicates 

 with the same tube, indirectly by the ventro-tegumentary vessels. According to Bourne 

 blood reaches the ventral vessel by the hearts ; of the truth of this there can be 

 hardly any doubt ; he considers^ however, that the ventro-integumentary vessels take 



PONTOSOOLEX. TRANSVERSE SECTION 



TO SHOW BLOOD-SYSTEM. 



(After Perrier.) 



V. d. Dorsal vessel, v. s. Ventral, v. i. Sab- 

 neural vessel. h. Intestinal plexus, v. t., b. 

 Tegumental system of vessels, t. TypMosole. 



