452 ZOOLOGY SECT. 



side unite to form a vas deferens or spermiduct (v. def ), right or 

 left as the case may be, which passes almost straight backwards to 

 open by the corresponding male aperture on the fifteenth segment. 



The female reproductive organs consist of a pair of ovaries, a 

 pair of oviducts with a pair of receptacula ovorum, and two pairs of 

 receptacula seminis. The ovaries (ov) are minute pear-shaped bodies, 

 which are situated in the thirteenth segment, attached to the 

 septum between the twelfth and thirteenth. The oviducts (ov. d) 

 are a pair of short tubes, each with a comparatively wide funnel- 

 shaped opening into the cavity of the thirteenth segment, and 

 extending backwards and outwards in the fourteenth segment to 

 open at the female aperture on the ventral surface of the latter. 

 The receptacula ovorum are a pair of reniform sacs which open into 

 the funnel-shaped ends of the oviducts. The receptacula seminis 

 or spermotheccB (rec) are two pairs of rounded sacs which open on 

 the exterior in the intervals between the ninth and tenth and tenth 

 and eleventh segments. 



Though hermaphrodite, the Earthworm is not self-impregnating, 

 but two individuals provide for mutual fertilisation by an act of 

 copulation. The copulating individuals apply themselves together 

 by their ventral surfaces, the heads pointing in opposite directions, 

 and become attached in this position by the setae of the genital 

 region and by a viscid secretion from the clitellum and of the 

 capsulogenous glands (p. 446), situated in the neighbourhood 

 of the reproductive organs. The sperms from the male apertures 

 of each pass along temporarily formed grooves to the receptacula 

 seminis of the other. 



When the ova are mature they are discharged from the ovary 

 into the cavity of the thirteenth segment, whence they pass out 

 to the exterior through the oviducts, to be enclosed in the cocoon 

 (vide infra), after having being detained for a time in the 

 receptacula ovorum. 



Development. The oosperms or fertilised ova of the Earth- 

 worm are enclosed, together with a quantity of an albuminous fluid 

 derived from the capsulogenous glands, in a cocoon, the wall of 

 which is formed of a viscid secretion from the glands of the 

 clitellum, hardened and toughened by exposure to the air. The 

 cocoon is deposited in the earth and the embryos develop into 

 complete, though minute, worms before they make their escape. 

 The segmentation is somewhat unequal. A flattened blastula 

 (Fig. 370, A) is formed, with a large but flattened segmentation- 

 cavity. This becomes invaginated to form a cylindrical gastrula 

 (B) ; the blastopore narrows and subsequently gives rise to the 

 mouth of the adult. A pair of large mesoderm cells are early marked 

 off from the other cells of the gastrula ; these undergo division 

 to form a pair of mesoderm bands (C, mes) composed of several 

 rows of small cells which grow forwards towards the mouth. 



