508 



ZOOLOGY 



body, and as they mark the middle (third) ring of all those segments 

 the extent of which can be checked by the nephridiopores, it is 

 legitimate to assume their segmental value in the anterior and 

 posterior regions, where the controlling excretory apertures are 

 absent. By the clue thus furnished it is found that there are six 

 segments in front of that bearing the first pair of nephridiopores, 

 and three behind that bearing the last pair, making a total of 

 twenty-six metameres : of these the first seven and the last three 

 have less than the normal number of rings. 



The anterior sucker bears on its dorsal surface five pairs of small 

 black spots, the eyes (e. 1, e. 5), the arrangement of which shows 



nsl n.s vs 



Fig. 405. — Hirudo medicinalis ; transverse section, b. t. botryoidal tissue ; c. to. circular 

 muscles ; cr. crop ; rr'. diverticula of crop ; cu. cuticle ; d. ep. epidermis ; d. s. dorsal sinus ; 

 d. v. /». dorso-ventral muscles ; I. m. longitudinal muscles ; I. v. lateral vessel ; n. c. nerve- 

 cord ; nph. 1 — h, nephridium ; n. s. nephrostomial sinus ; nst. nephrostome ; ts. testis ; v. d. 

 vas deferens ; vs. vesicle of nephridium ; v. s. ventral sinus. (After Marshall and Hurst.) 



them to be special modifications of sensory papillae, since they 

 occupy in the first five segments the precise position occupied in 

 the sixth and following segments by segmental papillae. 



The perfectly definite and comparatively small number of 

 metameres in the leech offers a striking point of contrast with 

 what we have met with in the Chsetopoda, and is to be looked 

 upon as a mark of higher differentiation. 



Body-wall. — The body is covered externally by a thin cuticle 

 (Fig. 405, cu.), which is constantly being cast off in patches and 

 renewed. Beneath it is an epidermis (d. ep.) consisting of hammer- 

 shaped cells, separated at their inner ends by spaces in which 



