4<6Q ALFRED CxIBBS BOURNE. 



through a new development, viz. botryoidal tissue, which may- 

 play an important role, forming a secondary coelom. 



Hirudo. 



The Lateral (longitudinal) Vessels. — These vessels 

 anastomose with one another anteriorly and posteriorly 



(fig. 5,/. t;.). 



They give rise to two sets of branches ; the one set arise 

 from the internal border, the other from the external, the 

 former divide almost immediately into two diverging branches 

 which anastomosing — those of the one side with those of the 

 other — upon the abdominal wall, beneath the ventral vessel, 

 establish along the whole length of the body a direct com- 

 munication between the two lateral vessels ; the trunks giving 

 rise to these anastomoses are eighteen in number on each side, 

 and have been called by Duges latero-abdominal branches 

 (fig. 5, /. ab.). 



The branches arising from the external borders of the lateral 

 vessels are alternately long and short ; the short are distri- 

 buted to the median lateral region, and may be called later o- 

 lateral branches (/. /.). The long branches (/. d., latero- 

 dorsal, Duges) passing dorsalwards divide into two branches 

 which both approach the median dorsal line, but never have, as 

 Cuvier pointed out, any direct connection with the dorsal 

 vessel (sinus a. g. b.); of the two branches one is anterior, the 

 other posterior. 



Those branches of the latero-dorsal vessels, situated anteriorly 

 to the gastro-ileal region, never anastomose from right to left 

 side. 



In the gastro-ileal region the posterior branches of the latero- 

 dorsal vessels follow the common law, but not so the anterior 

 branches; these latter anastomose, those of one side with those 

 of the other, forming arches above the gastro-ileal intestine. 



All these branches resemble the lateral vessels in the muscular 

 structure of their walls. 



Cutaneous Networks. — These originate from the above- 

 mentioned branches, and form three successive layers (the 



