CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 465 



The supernumerary vessel of De Quatrefages is in reality the 

 ventral vessel. The only point concerning which I am able to 

 speak from my own observations is with regard to the relations 

 of the vascular dilatations in the branchiae. 



These dilatations occur in the first pair of branchiae, and in 

 every third succeeding pair. 



Branches of the lateral vessel run into these and end by an 

 open mouth, the dilatation on the other hand communicates 

 with the lateral sinus which runs parallel to the lateral vessel. 



It seems to me quite clear that these branchial vascular 

 dilatations of Branchellion are represented by the rudimen- 

 tary dilatations described above in Pontobdella, Piscicola, 

 and Clepsine, and that probably we have here the only com- 

 munications between the " closed '^ vascular system and the 

 system of lacunar spaces or sinuses. 



It is unnecessary for me to recapitulate here the condition of 

 either system in these four genera, they present a marked unity 

 of type, all approaching more or less closely a condition in which 

 four longitudinal trunks communicate with one another and lie 

 respectively in four sinuses which fully communicate, and which, 

 or their branches, contain several of the principal organs. We 

 have, in fact, a vascular system on the one hand, and a coelomic 

 system upon the other, branches of the former opening into the 

 dilatations of the latter in the lateral region in the skin, thus 

 establishing a direct continuity of the lumen throughout. 



Gnathobdellid^. 



How slightly, but how regularly, the Gnathobdellidae vary 

 from the Rhynchobdellidse in this respect we shall now see. 

 The differences may be briefly summed up thus : 



1. In the Gnathobdellidse all trace of lateral sinus and of the 

 dilatations connected with it has vanished. 



2. All trace of the dorsal and ventral vessels has vanished. 



3. The lateral vessels with their connections and the dorsal 

 and ventral sinus system are placed in communication only 



