CONTRIBUTIONS TO THE ANATOMY OF THE HIEUDINEA. 455 



in the Hertwigs' sense, or may be something altogether unpro- 

 vided for in the artificial and valueless system of those authors. 



Rhynchobdellid^ : Pontobdella. 



We owe to Vaillant the most complete description of the 

 vascular system in this genus, Leydig having only made a few 

 isolated observations upon it.^ 



Vaillant^s observations, careful as they appear to be, are 

 based chiefly upon separate injections, which have resulted in 

 a complete mixture of the two systems. Vaillant desci'ibes (as 

 vessels) the dorsal sinus, the ventral sinus, and the lateral 

 vessels, but does not appear to have seen the dorsal vessels, 

 ventral vessels, nor the traces of lateral sinuses which exist. 



Dorsal and Ventral Sinuses. — In the anterior region 

 there is in Pontobdella a large sinus which surrounds the 

 pharynx and its sac, the brain, and nerve-cords, and in the 

 dorsal and ventral walls of which lie respectively the dorsal 

 and ventral vessels. Between the seventh and eighth post- 

 oral ganglia this becomes narrowed into the ventral sinus. 

 Anterior to this it gives off branches on either side, which, 

 uniting dorsally, form the dorsal sinus (fig. 63, d. s., d. v., v. s., 

 V. V. See also woodcut, fig. 2). 



The dorsal sinus encloses, for the greater part of its length, 

 the dorsal vessel. I say along the greater part of its length, 

 because here and there the tissue which is filling up the system 

 of sinuses has grown around the dorsal vessel, so that the latter 

 comes to lie side by side with the dorsal sinus. The same 

 remark applies to the ventral vessel and sinus. This arrange- 

 ment is carried to a greater length in Branch ell ion, and has 

 given rise to statements as to a double character of the dorsal 

 and ventral sinuses. 



The dorsal sinus tends to lose its character as a definite 

 sinus in the posterior region of the body, being split up into a 

 network of small sinuses. 



In the regions of the ganglia branches of the ventral sinus 

 1 ' Zeit. f. w. Zool.,' 1851, p. 319. 



