250 



COMPARATIVE ANATOMY 



CHAP. 



vascular system of the Hirudinea recalls that of the Nemertina. They 



also agree in the possession of blood sinuses. 



The vascular system of the Chcetopoda is strikingly different from 



that of the Hirudinea. It is entirely separate from the body cavity. 



Both the lateral vessels of 

 the Hirudinea are wanting. 

 The most important and 

 constant parts of the Chceto- 

 podan circulatory apparatus 

 are : (l^M^medio-dorsal 

 longitudin^Bj^ssel and (2) 

 a medio-venpil longitudinal 

 vessel (Fig. 165). The first 

 is mostly contractile ; in it 

 the blood streams from 

 behind forwards ; it lies 

 over the intestine, some- 

 times nearer the latter, 

 sometimes nearer the body 

 wall. The second is not 



FIG. 165. Transverse section through a Lumbricus, 

 diagrammatic. Ih, Body cavity ; eg, chloragogen cells ; contractile ; in it the blood 

 rm, circular musculature ; lm, longitudinal musculature ; gleams from before back- 

 dv, dorsal vessel ; ty, typhlosolis ; vt, typhlosolis vessel ; 

 np, nephridia ; vv, ventral vessel ; vln, lateral vessel of 

 the ventral chord ; bm, ventral chord with the neurochord 

 tubes ; vvn, sub-neural vessel ; bm, 2 setae of the ventral 

 row ; U, 2 of the lateral row. 



ward. It lies in the body 

 cavity, below the intestine 

 and above the ventral chord, 

 approaching sometimes the 

 one, sometimes the other. In the details of the arrangement, develop- 

 ment, and course of the vessels there is extraordinary variety, which 

 makes it impossible to describe them briefly and comprehensively. In 

 a very simple case the ventral vessel divides at the anterior end of 

 the body into 2 branches, which, surrounding the fore-gut, enter the 

 anterior end of the dorsal vessel. The dorsal and ventral vessels are 

 further connected in each segment by lateral vascular loops ; it is from 

 these especially that branches proceed to the body wall. Vessels 

 coming from a vascular network surrounding the intestine also enter 

 the dorsal vessel ; this network in many cases may be replaced by 

 a blood sinus lying between the epithelial and the muscular walls 

 of the intestine. The blood in the vessels of the Chcetopoda is generally 

 red, and contains colourless corpuscles. 



The following are brief descriptions of the. blood- vascular systems of an Oligochcetan 

 and of a Polychcetan (arbitrarily selected). 



Lumbricus (Fig. 166) (as an example of the Oligochceta). There are 5 longi- 

 tudinal vessels ; first a medio - dorsal vessel ; second and third 2 medio - ventral 

 vessels, one of which lies under the intestine and above the ventral chord, and 

 represents the ventral vessel which is always found in the Chcetopoda, while the 

 other is much finer and runs under the ventral chord, the former is known as the 

 ventral vessel, and the latter as the sub-neural vessel ; fourth and fifth, 2 delicate 



