VERMES BLOOD-VASCULAR SYSTEM 



251 



vd 



vessels which accompany the ventral chord throughout its whole length, running to 

 the right and left of it ; these are the lateral vessels of the ventral chord. The 

 sub-neural vessel is connected with the lateral vessels of the ventral chord at intervals 

 by transverse anastomoses. The dorsal 

 vessel, near the mid-gut, has segmental 

 swellings, so that it here assumes the 

 form of a string of beads. In the genital 

 segments it is connected with the ven- 

 tral vessel by 5 pairs of wider pouch-like 

 contractile vascular loops, the so-called 

 hearts. In the region of the mid-gut 

 the dorsal vessel gives rise in each seg- 

 ment to 3 pairs of vessels. The first 

 pair run laterally in the body cavity and 

 enter the sub -neural vessel. They give 

 off in their course vessels to the body 

 wall and the integument, and further, 

 in the lower part of their course, anasto- 

 moses to the ventral vessel and the 

 lateral vessels of the ventral chord. The 

 two posterior pairs run on the intestine 

 where they break up into an extremely 

 rich and close network. The typhlo- 

 solis and the muscular stomach are also 

 supplied from the dorsal vessel. An- 

 teriorly, between the third and fourth 

 pairs of hearts, there arises from the 

 dorsal vessel on each side a vascular 

 trunk, whose complicated branches sup- 

 ply the anterior part of the body, the 

 intestine, the body wall, the first pair 

 of Morren's glands, the pharynx, the 

 oesophagus, etc. ; these are also con- 

 nected with the ventral and sub -neural 

 vessels. In each segment the ventral 

 vessel gives rise to a lateral pair of ves- 

 sels, which branch in the body wall and 

 the integument. These branches anas- 

 tomose in that part of the body which 

 contains the stomach intestine with the 

 branches arising in each segment from 

 the first pair of lateral vessels of the 

 dorsal vessel, which latter maintain a 

 communication between the dorsal ves- 

 sel and the sub-neural vessel. The ventral and dorsal vessels give off branches to 

 the extreme anterior end of the body, which ramify in the body wall and pharynx. 

 The ventral vessel further divides at its anterior end into 2 branches, which pene- 

 trate to the brain and so form an oesophageal ring. The contractile part of the 

 blood-vascular system possesses muscular walls. 



Nephthys scolopendroides (Fig. 167) may be taken as an example of a Polychcctan 

 with homonomous segmentation of the body. We find here the two most important 

 typical vessels, the dorsal and the ventral. Both lie close to the wall of the 



FIG. 16*5. Anterior portion of the body of 

 Lumbricus terrestris, opened, to show the vascu- 



sperm sacs ; ph, pharynx ; aft, seminal vesicles ; vd, 

 dorsal vessel ; vv, ventral vessel ; vsn, subrieural 



vessel; vln > lateral vessels of the ventral chord; 

 bun, ventral chord; h, contractile vascular loops 



(hearts) between the dorsal and ventral vessels. 



