EARTHWORM 93 



and one to the left, on the ventral surface of segment 15. From 

 the external opening each of these sperm ducts continues forward 

 internally to segment xii, where it divides, one branch of each 

 running to the testis sacs in x and xi. In these sacs the sperm 

 ducts end in ciliated, funnel-shaped openings adapted for drawing 

 the sperm into the ducts, from which the latter pass to the exterior, 

 as noted above. 



Female Organs of Reproduction. These consist of a single 

 pair of ovaries situated ventrally in xiii. The eggs which de- 

 velop in them are set free directly into the cavity of the coelom 

 in segment xiii and pass into the oviducts which open internally 

 in this segment. The external opening of each oviduct is through 

 the ventral body wall of segment xiv. Each oviduct begins in xiii 

 as a ciliated, funnel-shaped opening situated near the ovary of 

 that side. The eggs may remain for a time in an enlarged anterior 

 portion of the oviduct, the ovisac, before leaving the body. 



The clitellum, which has been mentioned earlier, is a differ- 

 entiated glandular structure present in the body wall, normally 

 situated between segments xxxii and xxxvii. It serves as an 

 accessory organ during reproduction by the secretion of material 

 which forms a case in which the embryos develop, as described 

 below. 



In addition, the Earthworm possesses two pairs of seminal 

 receptacles attached laterally and close to the posterior septum 

 of segments ix and x. These open directly to the exterior through 

 the ventral body wall in the grooves between segments ix/x, 

 and x/xi. The seminal receptacles remain empty most of the 

 year, but at the breeding season when the worms pair, they are 

 filled with sperm received from another worm, and it is this foreign 

 sperm that is used to fertilize the eggs. 



Functional. The breeding season begins early in the spring. 

 At this time the seminal vesicles of each worm are well-filled with 

 mature sperm which have been developing since the previous breed- 

 ing period. In order to bring about an exchange of sperm, two 

 worms come into contact, with the head ends pointed in opposite 

 directions, in such a way as to bring the openings of the seminal 

 receptacles of one worm on segments ix/x in close apposition to the 

 ventral surface of the clitellum of the other worm. The mucus 

 glands in the skin between segments ix and xxxvii now become 

 active and secrete a mucus envelope, or slime tube, covering the 



