PHYLUM ANNULATA 



471 



371, nst) in connection with the testis: comparison with other 

 Hirudinea shows that this dilated end of the nephridium re- 

 presents a nephrostome which has lost its open funnel-like end 

 in correlation with the absence of a distinct coelome. The cells 

 of the botryoidal tissue appear to carry waste matter to the 

 nephridia. 



There is a complex vascular system, containing, like that of 

 the Earthworm, red blood, the plasma coloured with haemoglobin 

 and containing sparsely distributed colourless corpuscles. But a 



Diking difference from the preceding annulate types is found in 



FIG. 375. Xephridium of Hirudo medicinalis. a. 1. apical lobe ; m. I. middle lobe : 

 np. iiephridiopore ; nst. nephrostome ; r. 1. recurrent lobe ; t. I. testis lobe ; vs. vesicle 

 vs. d. vesicle duct. (After Bourne.) 



the fact that the blood-containing spaces are of two kinds blood- 

 vessels proper, having muscular walls ; and blood-sinuses, the walls 

 of which are devoid of muscle. 



The two principal blood-vessels are lateral in position (Figs. 

 373 and 376, /. v.\ running fore and aft at the level of the middle 

 of the nephridia and uniting with one another at the anterior and 

 posterior ends of the body. They send off branches both dorsally 

 and ventrally, some of which anastomose with one another. The 

 ultimate branches break up into capillaries in the integument, 

 nephridia, &c. 



The two principal sinuses are respectively dorsal (d. s.) and 

 ventral (v. s.), the former lying just above the enteric canal in the 

 middle dorsal line, the latter occupying a similar position on the 



