252 



COMPARATIVE ANATOMY 



CHAP. 



intestine. From the posterior end of the pharynx to the end of the body the dorsal 

 vessel gives rise in each segment to a pair of lateral vessels, which run to the body 

 wall and especially to the dorsal branch of the parapodia and there ramify. Soon 



t after its rise out of the dorsal vessel 



each lateral vessel gives off a branch 

 to the enteric wall, this branch there 

 forming a rich vascular network. At 

 the foremost end of the mid-gut the 

 dorsal vessel shows a slight swelling ; 

 it then leaves the intestine and runs 

 along the inner side of the body wall 

 in the region where the pharynx lies 

 to the extreme anterior end of the 

 body. It here divides into 2 branches, 

 which run backwards, to pass into the 

 anterior end of the ventral vessel at 

 the posterior end of the pharyngeal 

 region. The departure from the 

 general plan of the principal vessels 

 in the pharyngeal region is evidently 

 caused by the great development of 

 the pharynx. If they lay as close to 

 FIG. 167. Part of a transverse section through the pharynx as they do to the intes- 

 a segment of the body of Nephthys, diagrammatic, tine in the rest of the body) they 



to demonstrate the arrangement of the blood-vessels , , , , , , ., 



would be broken by its protrusion, 

 (partly after Jaquet). dp, Dorsal ; vp, ventral para- J l 



p<>dium, with bundles of sete and supporting seta; vd, The ventral vessel gives rise in each 

 dorsal vessel ; vv, ventral vessel ; gin, lateral vessels segment to a pair of lateral vessels, 

 of the ventral chord ; vg, connecting vessel between AV hich run along to the base of the 

 the dorsal and ventral parapodia; a, intestine; Im, ventral branc l les O f t he parapodia, 

 ventral chord ; k, gills ; kr/, branchial vessels. ... 



passing into the latter and there rami- 



the lateral branches of the dorsal vessel and the 



fying. A connection between 

 lateral branches of the ventral vessel takes place in the following way. Between 

 the dorsal and ventral branches of a parapodium there runs on each side a vessel 

 which ramifies on one side in the dorsal branch, on the other in the ventral branch. 

 These ramifications of the connecting vessels anastomose with the ramifications of 

 the dorsal or ventral lateral vessels. On the ventral side of each dorsal parapodial 

 branch a sickle-shaped gill is inserted. Two vessels from the vascular system of the 

 dorsal parapodial branch enter the gill, and with many windings and anastomoses 

 run through its whole length. Besides the dorsal and ventral vessels, Nephthys 

 possesses also two extremely delicate lateral vessels of the ventral chord, which are 

 connected by fine branches with the lateral branches of the ventral vessel. 



This arrangement of the blood-vascular system can, however, only to a very limited 

 extent be considered typical of the Polychccta. In the different families, indeed, very 

 great variations are to be found. The blood-vascular system is extraordinarily plastic. 

 It follows the smallest modifications in the structure and arrangement of the other 

 organs of the body. Its special form is above all dependent on the arrangement and 

 development of the gills. In the Tercbellidcc the dorsal vessel forms in the anterior 

 part of the body a tubular pulsating heart, which is narrowed anteriorly, and which 

 supplies the gills lying on the most anterior body segments with blood-vessels. At 

 its posterior end it splits into two branches, which encircle the intestine. These 

 branches unite below the intestine to form a sub-intestinal vessel which runs a 

 certain distance backwards over the ventral vessel. Then the sub-intestinal vessel 



