38 THE LEECH 



5. External openings. 



a. The mouth is a funnel- like depression in the anterior 



sucker, at the bottom of which are the three 

 jaws. 



b. The anus is a very small aperture on the dorsal 



surface in, or just in front of, the constriction 

 separating the posterior sucker from the body. 



c. The genital apertures. The leech is hermaphrodite : 



the male and female apertures are separate, and 

 are both on the ventral surface in the median line. 



i. The male aperture is a small hole with tumid 

 lips at the hinder border of the twenty-fourth 

 annulus, i.e. in the second annulus of the 

 sixth somite. From it the muscular penis is 

 often seen protruding. 



ii. The female aperture is less conspicuous than 

 the male, and is placed one somite further 

 back, i.e. at the hinder border of the twenty- 

 ninth annulus, the second annulus of the 

 seventh somite. 



d. The apertures of the nephridia, or excretory organs, 



are minute paired openings on the ventral surface 

 of the body, in the hindmost annulus of each 

 somite from the second to the eighteenth in- 

 clusive. 



FIG. 13. Hirudo medicinalis, seen from the ventral surface. (After A. 

 ' G. Bourne.) 



The numbers 1 to 23 indicate the somites ; the numbers I. to Y. the five 

 annuli of the twelfth somite. 



A, anterior sucker, with the mouth. B, posterior sucker. G, male aperture. 

 H, female aperture. N, nephridial apertures. 



FIG. 14. Hirudo medicinalis. A diagrammatic figure of the renal, ner- 

 vous, and reproductive systems. The animal has been opened along 

 the mid-dorsal line, pinned out, and the alimentary canal removed. 

 (After A. G. Bourne.) 



The somites are numbered 1 to 23, and their boundaries are indicated by 

 the dotted lines. 



CGr, supra-03sophageal ganglion. EP, left vesicula seminalis. G 1, tho 

 first of the twenty-three post-oral ganglia. GL, glandular enlargement of the 

 oviducts. L, lateral vessel. LA, latero-ventral branch of lateral vessel. LD, 

 latero-dorsal branch. LL, latero-lateral branch. N" 1, the first of the seven- 

 teen nephridia of the left side. O, nerve collar. OV, ovisac containing 

 the left ovary. PE, penis. T 4, the third testis of the left side. VD, vas 

 deferens. 



