70 OLIGOCHAETA 



vessel in its earliest stage is simply a sKght thickening of the splanchnopleure ; for 

 some time it remains solid and only subsequently becomes hollow. In the view of 

 some writers, for example Wilson, the ventral vessel is from the first hollow and its 

 cavity is the remains of the segmentation cavity. 



In many of the lower Oligochaeta the ventral vessel terminates^ in front by 

 bifurcating and ultimately joining the dorsal vessel by a loop round the gullet ; in 

 Chaetogaster however it ends in a solid cellular cord which bends upward and ends 

 freely in the neighbourhood of the cerebral ganglia. In the higher Oligochaeta the 

 ventral vessel branches anteriorly and communicates with the dorsal vessel only through 

 a capillary system. In two Tubificidae the ventral vessel terminates anteriorly in an 

 altogether peculiar fashion. In Lophochaeta the ventral vessel appears to end in the 

 eighth segment in a pair of hearts which communicate with the supra-intestinal trunk ; 

 but from the angle formed by the bifurcation an extremely slender vessel passes forward 

 and after receiving three branches from the sub-intestinal vessel joins that vessel in 

 the seventh segment ; the single vessel formed by the fusion of the two terminates 

 anteriorly in the usual way. In Bothrioneuron the narrow continuation of the 

 ventral vessel commences in segment vii., and without receiving any branches from 

 the sub-intestinal joins it in segment vi. These facts were described and figured by 

 Stolc (3), whose figures are here reproduced (woodcuts, figs. i6, i8). 



The Sub -intestinal vessel. — I apply this term to the usually paired vessels 

 called by Perkier and Bourne ' Intestino-tegumentary ' vessels, and by Benha.m 

 ' lateral longitudinal ' vessels. They are first seen in the family Tubificidae ; in 

 Lophochaeta and in Bothrioneuron where their relation to the ventral vessel has 

 already been referred to. In Lophochaeta the ventral vessel divides in the seventh 

 segment into two trunks, one lying above the other : the uppermost of the two is 

 closely applied to the ventral wall of the alimentary canal and appears to be intimately 

 concerned with its blood supply ; it has much the same relations to it below that the 

 supra-intestinal vessel has above ; the same vessel occurs in Bothrioneuron, but it 

 begins earlier, in the sixth segment. In the higher Oligochaeta this vessel, or at any 

 rate the vessel which I regard as its equivalent, is invariably double — with the possible 

 exception of the Lumbricidae ; Howes in his ' Biological Atlas ' has figured in that 

 worm a single sub-intestinal vessel which Jackson (' Forms of Animal Life ') considers 

 to be non-existent, but to have been mistaken for the impression of the attachment 

 of the mesentery. In many other earthworms there are a pair of these vessels which 

 arise from the intestinal plexus ^ and run for a short distance closely attached to the 



1 In Benhamia schlegelii Horst iigures and describes these vessels as originating from the dorsal trunk 

 just in front of the antepenultimate pair of hearts. 



