IV 



VERMES BLOOD-VASCULAR SYSTEM 



249 



dm. 



Hirudinea. In describing the body cavity we have already drawn 

 attention to the difficulty of distinguishing it from the blood-vascular 

 system. We are, besides, not certain that these are two originally 

 separate systems, and it 

 seems almost necessary to 

 consider the whole to- 

 gether. There occur al- 

 most universally 4 longi- 

 tudinal vessels (Fig. 163) 

 1 dorsal, lying over the 

 intestine, 1 ventral, in 

 which the ventral chord 

 lies, and 2 lateral, which 

 in many cases pulsate. Of 

 these 4 vessels the ventral 

 one (ventral sinus) may 

 best be considered as the 



, ,. -, -j FIG. 163. Transverse section through Hirudo, diagram- 

 principal part 01 a reduced matic> m> C ircular;musculature ; Im, longitudinal muscular 

 body cavity. The dorsal layer ; vl, lateral vessels ; np, looped canals (nephridia) ; vd, 

 Vessel is wanting in Ne- dorsal vessel ; ^ m > dorso-ventral musculature ; enp, terminal 

 vesicle of the nephridia ; 6m, ventral chord ; vv, ventral 

 pliehs and some land ves sel ; A, testes ; vd, vas deferens ; md, mid-gut. 



leeches. The longitudinal 



vessels are connected together, chiefly at the anterior and posterior 

 ends of the body, by fine vascular branchings. Such a connection also 

 takes place in various ways in other parts of the body. The peripheral 



vascular system consists principally of 

 2 well -developed systems of branched 

 and often anastomosing capillaries, one 

 of which lies in the integument and 

 penetrates into the body epithelium, the 

 other spreading out over the intestine. 

 The excretory and sexual organs are 

 richly supplied with blood-vessels. In 

 -w Nephelis (Fig. 164) and land leeches 

 there are, in connection with the anas- 

 tomoses between the lateral and ventral 

 vessels, ampullae or blood vesicles in 

 segmental arrangement, one on each side 

 (land leech) or two together (Nephelis). 



FIG. i64.-vascuiar system in 4 I n Branchellion on each side, at the base 

 segments of the middle part of the of every third gill, there is a blood sinus, 



Ventral vMsef^S lateraf Basel's *a' ^^6116(1 i nto a Vesicle. The blood 6V6ry- 



ampuiise. ' where contains colourless amoeboid cor-, 



puscles, and often free nuclei. In the 



Gnathobdellidce the blood is red. Haemoglobin is found dissolved in 

 the blood plasm. 



By the presence of one dorsal and two lateral vessels, the blood- 



