CCELOM AND VASCULAR SYSTEM IN THE LEECH. 4P1 



body-wall. The dorsal branches of tlie lateral vessels pass into 

 small annular vessels communicating with the plexus of minute 

 capillaries in the epidermis. From these, again, arise capil- 

 laries going to small sinuses which run into the lateral trans- 

 verse sinuses, and so into the dorsal sinus. 



Similarly the ventral sinus sends annular sinuses along the 

 ventral region of the body-wall opening into the epidermal 

 plexus, whence arise capillaries joining the latero-abdominal 

 vessels. 



Continuity between the two systems has also been shown to 

 take place by means of capillaries on the wall of the alimentary 

 canal, and probably exists on the other internal organs of the 

 body. 



Two questions still remain to be solved : firstly, as to the 

 circulation of the haemolymph; secondly, as to the exact 

 homology of the channels in which it flows. 



With respect to the first of these problems, I have no direct 

 observations to record ; but it may be pointed out that the 

 presence of the valves described above show, at least, that the 

 hsemolymph must flow in a constant direction — that there is a 

 real circulation, not a mere motion backwards and forwards. 

 It seems to me extremely probable that the annular vessels 

 collect the oxygenated blood from the epidermal plexus, and 

 carry it into the latero-dorsal and latero-lateral vessels, whence 

 it would be pumped into the lateral vessels. From these some 

 of the hsemolymph must be carried by the latero-abdominal 

 vessels to the various organs of the body, and to the ventral 

 cutaneous plexus. The annular sinuses would collect it from 

 this plexus and carry it into the ventral sinus. The abdomino- 

 dorsals and the dorsal sinus would appear to supply the dorsal 

 and lateral cutaneous plexus. 



We are left in considerable uncertainty as to the true nature 

 of some of the spaces. That the lateral vessels belong to the 

 real vascular system, and that the ventral sinus and perine- 

 phrostomial sinuses belong to the true coelomic system, seems 

 to be clearly established both by comparative anatomy and by 

 the embryological researches of Burger (2). This observer, 



VOL. 42, PART 4. — NEW SERIES. LL 



