THE ANATOMY. VASCULAR SYSTEM 



71 



Fig. 17. 



ventral surface of the gut ; later however they leave the walls of the intestine and 



run freely suspended in the body cavity ; they end anteriorly in a network of capillaries. 



These vessels occur in Megaacolex (BOURNE), Pontoscolex (PERKIER) (woodcuts, figs. 



20, 21), Eudrilus (BEDDARD), and other forms. In Eudrilua 



the sub-intestinal vessel is partly double and partly single 



like the dorsal vessel of other earthworms ; it bifurcates 



in the neighbourhood of the calciferous pouches and is 



single between them. In another Eudrilid, Libyodrilun, 



I have traced the course of these vessels for a little 



distance ; they are double all the way and give off two 



pairs of branches in each of the segments through which 



I followed them ; one pair of branches go to the septum, 



the other to a muscle suspending the intestine ; numerous 



minute twigs connect them with the intestinal plexus. 



In Megascolex coeruleus the vessel in question only extends 



back to the thirteenth segment ; after this there is a small 



vessel in each segment running from intestine to septum 



which is its equivalent. 



The Sub-nervian vessel. This is absent in the lower 

 Oligochaeta and is by no means always present in the 

 earthworms. It exists in Lumbricus, Penchaeta, and 

 some other genera, and lies, as its name implies, beneath 

 the nerve cord ; it is even sometimes partially imbedded 

 in the ventral body-wall. The presence or absence of this 

 vessel served CLAPAREDE with one of his characters for 

 separating the Limicolae from the Terricolae. 



3. Commissural vessels. 



The dorsal vessel in all Oligochaeta is connected with 

 the ventral vessel by circular trunks which run round 

 the alimentary canal. The simplest and most primitive 

 arrangement of the vessels appears to be that which 

 characterises the families Tubificidae, Phreoryctidae, and 

 Lumbriculidae. In these worms there are at least one pair 



of such vessels in each segment of the body the regular metamerism of the vascular 

 system being thus very apparent. In the anterior region of the body one or more of the 

 commissural vessels are specialized and become contractile, being then usually termed 



SPARGANOPHILUS. GENERAL 

 ANATOMY. 



(After Benham.) 



1, 4, 6. Perivisceral vessels. a, j, 7 



Dorstil vi .<,>!. ... S|-rm:ithri n. S. 



Sperm-sacs. 9. Inteetino-tegnmen- 

 tary vessels. 10. Ovary, n, 12. Dorso- 

 aml ventro-tejfumentary trunks. 



