208 



MANUAL OF ZOOLOGY 



SECT. 



globin and containing sparsely distributed colourless cor- 

 puscles. But a striking difference from the preceding annelidan 

 types is found in 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. 112 and 113, /. z/.), running fore and aft at the level of 



-its 



FIG. 113. Diagram of principal blood-channels of Leech, d. s. dorsal sinus, 

 /. v. lateral vessel ; v. s. ventral sinus containing nerve-cord. 



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 



