166 



Annelida. 



specially found on tlie anterior part of the body^ but sometimes 

 also on otter segments. Auditory vesicles are more rarely 

 present. The vascular system (Pig. 130) is usually very 



well developed ; there 

 A B C isj as a rule, a longi- 



tudinal vessel on the 

 dorsal side, the dorsal 

 vessel, and a similar 

 one on the ventral side, 

 the ventral vessel; 

 these are united by 

 transverse vascular 



arches. The dorsal 

 trunk, sometimes, also,, 

 some of the transverse- 

 vessels, is pulsatile, and 

 performs the functions 

 of a heart; the blood- 

 stream is from behind, 

 forwards in the dorsal 

 vessel; in the ventral 

 vessel in the opposite 

 direction. From these- 

 trunks smaller branches 

 go to different parts, to 

 the gut, etc., also to the 

 gills when these are 

 present. The vascular 

 fluid is usually coloured ; 

 as a rule red, sometimes yellow or green. The vascular system in some 

 forms (Chaetopods) is completely separated from the body-cavity 

 which contains a special colourless fluid. In other cases, e.g., in Leeches,, 

 this system communicates with the body-cavity, which, moreover, 

 is here of small extent, and modified to form vascular sinuses.*" 

 In certain Chaetopods the vascular system is entirely wanting. 



Fig-. 129. Nervous system of different Chaetopods 

 (B Serpula, Aphrodite). c cerebral ganglion, g 

 ventral ganglia, o eye. — After Quatrefagea. 



Pig. 130. Anterior end of an Annelid, with alimentary canal and vascular 

 system figured. Diagrammatic, m mouth, i- dorsal vessel, b ventral vessel, p pulsatile 

 transverse vessel. — Orig. 



* The opinion has recently been advanced, that in Leeches there is no communica- 

 tion bet-ween the vascular system and the spaces -w-hicli represent the body-oa-vity. 



