THE STRUCTURE OF THE EARTHWORM. 197 



ated one on each side of every segment except the first, and 

 attached to the posterior mesenteric septum of the segment. 

 Each canal communicates internally, by a wide funnel-shaped 

 ciliated aperture, with the perivisceral cavity, while external- 

 ly it opens by a minute pore, which is usually close to the in- 

 ternal pair of seta?. 1 



A colorless fluid, containing colorless corpuscles, and an- 

 swering to the blood of other invertebrated animals, occu- 

 pies the perivisceral cavity ; but, in addition to this, there is 

 a deep-red fluid, devoid of corpuscles, which fills a very large- 

 ly developed system of pseud-haemal vessels. These consist 

 of longitudinal and transverse principal trunks, and of very 

 numerous branches which proceed from them and ramify in 

 all parts of the body, except the cuticle and hypodermis. 



The longitudinal trunks are three : one supra-intestinal^ 

 which lies along the dorsal aspect of the alimentary canal ; 

 one sub-intestinal, which corresponds with this on the ven- 

 tral aspect of that canal ; and one sub-neural, which lies be- 

 neath the ganglionic cord. 



The supra-intestinal and sub-intestinal vessels are con- 

 nected in the greater number of the segments by pairs of com- 

 missural transverse trunks, which embrace the intestine, and 

 give off numerous branches to it. The supra-intestinal and 

 sub-neural vessels give off transverse trunks into the mesenter- 

 ic septa, which branch out into the muscular layers, and some 

 of which anastomose so as to form a second set of transverse 

 communications. Moreover, the sub-neural trunk and the 

 sub-intestinal trunk respectively send branches to each seg- 

 mental organ, upon which they are distributed, and, anastomo- 

 sing, give" rise to another series of communications between 

 the longitudinal trunks. 



In the seven most anterior segments, the longitudinal 

 vessels break up into a network, and there are no distinct 

 transverse commissural vessels. Behind these, and in the 

 region of the generative apparatus, the commissural vessels 

 are greatly dilated, and form from five to eight pairs of so- 

 called hearts which are attached to the anterior faces of as 

 many mesenteries. These contract from the dorsal toward 

 the ventral side. 



The nervous system consists of two cerebral ganglia 

 lodged above the pharynx in the third segment, and united 



1 Gegenbaur, " Ueberdie sogenanntenRespirationsorgane des Regenwurms." 

 (Zeitschriftfur wiss. Zoologie, 1852.) 



