230 PHYLUM ANNELIDA. 



bearing nephridia the terminal lobe lies close upon and 

 dorsal to a testis, but there is no morphological meaning in 

 this approximation. Each consists of two parts, a twisted 

 horseshoe-shaped glandular region, where the actual ex- 

 cretory function is discharged, and a spherical, internally 

 ciliated bladder opening to the exterior. Within the latter 

 there is a whitish fluid with numerous waste crystals. The 

 nephridia secrete a clear fluid which helps to keep the skin 

 moist, and thus makes respiratory diffusion easier. 



Reproductive system.- -The leech, like many other 

 Invertebrates, is hermaphrodite, containing both male and 

 female reproductive organs. The essential male organs or 

 testes are diffuse, being represented by nine pairs, lying on 

 each side of the nerve-cord in the middle region of the 

 body. Each is a firm globular body, within which mother 

 sperm cells divide into balls of sperms. The spermatozoa 

 pass from each testis by a short canal leading into a wavy 

 longitudinal vas deferens. This duct, followed towards 

 the head, forms a coil (so-called seminal vesicle) as it 

 approaches the ejaculatory organ or penis. From the coil 

 on each side the sperms pass into a swollen sac at the base 

 of the penis, where, by the viscid secretion of special 

 ("prostate") glands, they are glued together into packets 

 or spermatophores. These pass up the narrow canal of 

 the muscular penis, and leave the body on the middle 

 ventral line between rings 30 and 31, when they are trans- 

 ferred in copulation to the female duct of another leech. 



The female organs are more compact. The two small 

 tubular and coiled ovaries are enclosed in spherical vesicles, 

 the walls of which are continued as two oviducts, which 

 unite together in a convoluted common duct. This is 

 surrounded by a mass of glandular cells, which exude a 

 glairy fluid into the duct. Finally, the duct leads into a 

 relatively large muscular sac the " uterus ' : -which opens 

 through a sphincter muscle on the middle ventral line 

 between rings 35 and 36. 



The favourite breeding-time is in spring. Two leeches in- 

 seminate one another, uniting in reverse positions, so that the 

 penis of each enters the uterus of the other. Spermatophores 

 are passed from one to the other, and the contained sperms 

 may remain for a long time within the uterus, or, liberated 



