rs6 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the fact that a group of five of these annuli corresponds to a single 

 complete somite of a Polychaete or Oligochaete. 



The leech is hermaphrodite, the gonad being formed from the lining 

 of little spherical coelomic chambers — nine (sometimes ten or eleven) 

 pairs of testes (Fig. 73, B, t) situated ventrally on each side of the mesial 

 plane, and a single pair of rather smaller rounded ovaries (ov) situated 

 further forward. Ovaries and testes occur in successive somites, i.e. 

 at a distance of five annuli from one another. As regards ducts, each 

 ovary is prolonged into a slender oviduct (Fig. 73, C, od) — one or other 

 of which passes under the ventral nerve-cord. The two ducts unite to 

 form an unpaired oviduct which winds from side to side in the substance 

 of a transparent-looking albumen-gland (a.g) and on emerging from this 

 bends forward on itself and is continued as the thick-walled muscular 

 vagina (z)) to the median external opening. The eggs before being laid 

 accumulate in the vagina and the inner portion of this organ is conse- 

 quently sometimes termed the uterus. 



Each testis is continued outwards into a minute vas efferens (Fig. 73, 

 B, v.e) and these open at right angles into a longitudinal duct — the vas 

 deferens (v.d). This extends forward to the level of the somite next 

 in front of the ovaries where its lining becomes highly glandular. This 

 glandular portion of the vas deferens is coiled into a compact mass — 

 the epididymis (ep). Beyond this its wall becomes thick and muscular 

 (ductus ejaculatorius — e.d) : it then is continued towards the middle 

 line as a very narrow tube and opens into the swollen inner end 

 (" prostate ") of a thick muscular tube — the penis — which can be pushed 

 out through the male opening. This like the female opening is unpaired 

 and in the mid-ventral line, the two openings being five annuli ( = one 

 somite) apart. 



The eggs of the leech, fertilized in the vagina or uterus, are deposited 

 in a cocoon, measuring about 25 mm. by 15 mm., secreted on the surface 

 of the somites in the neighbourhood of the female opening, though this 

 portion of the ectoderm is not so thickened as to form a conspicuous 

 projecting clitellum as is the case in the earthworm. A thick outer 

 spongy layer of the cocoon is said to be deposited secondarily by the 

 action of the lips and to be formed possibly by the pharyngeal glands. 

 The cocoon is deposited in damp earth near the water margin. 



The nervous system of the leech is arranged on the same general 

 plan as that of Lumbricus, the only important difference being that the 

 ganglia at the two ends of the ventral nerve-cord are crowded together 

 and fused. Thus the circum oesophageal commissures pass ventrally 

 into a ganglionic mass representing the first five ganglia of the ventral 



