ANELLATA. 145 



(b), which unite into a single longer tortuous tube (c), terminating 

 in an expanded fusiform uterus (d), which opens externally at the 

 twenty-ninth segment. The ovaria contain numerous round cor- 

 puscles, in which there are several germinal vesicles ; the parietes of 

 the uterus present both longitudinal and transverse muscular fibres. 



The fertile ova of the medicinal leech are discharged in groups 

 of from six to fourteen, enveloped in a nidus or cocoon of mucus. 

 The cocoon is ovate, two thirds of an inch in length and half an 

 inch in diameter. It has a rough outer surface, but is smooth 

 and slightly tuberculate within. In the month of August conical 

 excavations may be observed in the slime at the sides of the reservoir^ 

 in each of which there is a cocoon. In a few days after the ova have 

 been thus expelled and protected, the young leeches are extruded. 

 The formation of the cocoon has been observed by Dr. Johnson in 

 the rivulet leech (Hirudo vulgaris). In this species, when a cocoon 

 is about to be formed, the body is observed to be greatly contracted 

 both above and below the uterus ; the included part swells, then be- 

 comes milky white, from the formation of a film into which the animal, 

 having attached itself by its anal sucker, forces, with some effort, the 

 whole contents of the uterus. This being done, the leech elongates 

 the anterior part of the body, and thus loosening the enveloping mem- 

 brane, withdraws its head as from a collar. It sometimes bends back 

 its head, and, drawing the collar forwards, gently aids in its removal. 

 The j)rocess generally occupies about twenty minutes. The cocoon 

 is at first very elastic, and has no determinate figure. After the leech 

 has attached it to some adjoining substance, it fashions it with its 

 mouth into an oval form. The points of the cocoon from which the 

 leech withdrew its head are weaker than the rest, and from these the 

 young escape. 



The earth-worm, like the leech, is androgynous : the testes are four 

 in number, of a subglobular figure, granular texture, and opake white 

 colour : their ducts open upon the external surface. Two other parts 

 are to be reckoned among the accessory organs of the male sex ; the 

 first are the imperforated penes or stimulating filaments, which may be 

 developed from the thirty-second to the thirty- eighth ring inclusive ; 

 their base adheres intimately to the cellular tissue : they have no 

 communication with the genital apertures, are developed only at the 

 breeding season, and are deciduous. The second accessory organ 

 is that thickened part of an earth-worm which is situated between the 

 thirtieth and the fortieth segments : it is called the clitellum, and 

 when two earth-worms are disturbed, the adhering clitella are the 

 last parts to give way. The ovaria are eight in number, arranged 

 four on each side. The ducts of each lateral series unite to form a 



