C(ELOM AND VASCULAR SYSTEM IN THE LEECH. 485 



constricted, being provided with sphincter muscles (fig. 16), 

 and on its inner side, within the lateral vessel, is a valvular 

 arrangement composed of a mass of long-stalked cells situated 

 round the aperture. Although it is somewhat difficult to 

 judge certainly as to the action of these valves, yet I think 

 there can be little doubt that they prevent the hsemolymph 

 from returning into the dorsal branches when the lateral vessel 

 contracts. For the bunches of cells would block up the narrow 

 opening if the fluid tended to return into these branches, as in 

 the case of similar valves in other annelids. On the other 

 hand, on the hsemolymph flowing into the lateral vessel, the 

 valvular cells would merely hang freely in its wide lumen. 



The only other branches coming from the lateral vessels are 

 the latero-abdominal vessels of Duges, arising about 

 midway between the dorsal branches (figs. 2, 3, and 5). They 

 bend dovvnwards and bifurcate, each branch joining its fellow 

 from the opposite side below the ventral sinus. A lozenge- 

 shaped figure is thus formed by the right and left branches, 

 generally more regular than in fig. 3. The lozenges are 

 joined together by short median vessels. 



The latero-abdominal vessels give off" branches supplying 

 the nephridia, and the capillary cutaneous plexus of the 

 ventral and ventro-lateral regions. 



There are no valves round the aperture of the latero- 

 abdominal into the lateral vessel, and the entrance is not much 

 constricted. 



This description of the main trunks of the contractile 

 vascular system agrees in all essential points with that of 

 Gratiolet (4), excepting for the valves described above, which 

 apparently were missed by previous observers. 



Gratiolet would seem to have been mistaken in thinking 

 that the anterior branches of the latero-dorsal vessels joined 

 across to form a complete arch above the dorsal sinus only in 

 the region of the intestine ; this occurs also in the region of 

 the sacculated crop,^ and here, as elsewhere, these arches give 



• It is possible that individual leeches vary in this respect, since Jaquet (5) 

 also states that there is no union from side to side in the anterior region. 



