l86 THE INVERTEBRATA 



into the uterus for storage. In the Trematoda the ductus communis 

 is long and coiled and serves for the storage of eggs. It is called the 

 "uterus '\ but it is not of course homologous with the "uterus " of the 

 Rhabdocoelida which will be described shortly, nor with the " uterus " 

 of the Cestoda which is again probably a different organ. 



The genital atrium receives not only the openings of the male and 

 female organs but also certain accessory organs. In the Rhabdocoelida, 

 of which Mesostoma is an example, there open out from the genital 

 atrium on either side the paired uteri (Fig. 159, i), in which the eggs 

 are stored before laying. In Dalyellia (Fig. 147) the fertilized eggs 

 pass into the parenchyma. There is another opening which leads into 

 a short muscular receptacle, the bursa copulatrix. The bursa copulatrix 

 receives the penis of another individual during copulation. Sperm 

 is deposited here but remains only for a short time before being 

 expelled by muscular contractions and received into the ovidufct 

 where it is collected near the ovary in the true receptaculum seminis. 

 In the Tricladida the uterus and the bursa copulatrix are replaced 

 by organs, the homologies of which are doubtful. These are the 

 unpaired stalked gland organ and the unpaired muscular gland organ. 

 The stalked gland organ is often called the "uterus" but it has not 

 been observed to contain eggs. It is regularly present, whereas the 

 muscular gland organ is often absent. It has recently been shown that 

 the stalked organ serves as a bursa copulatrix and receives tem- 

 porarily the penis and the sperm of another individual. 



During copulation the ventral surfaces of two animals are applied 

 together so that the genital openings lie opposite to each other. The 

 penes are extruded through the genital opening of one copulant into 

 the genital opening of the other. There is a mutual exchange of sperm. 

 Since the ova are ripe at the same time as the sperm, and as, in many 

 forms, there is only one common genital opening to the exterior, 

 special precautions are necessary to prevent self-fertilization. To 

 ensure that cross-fertilization shall take place a great elaboration of 

 the structures surrounding the genital atrium has occurred, resulting 

 in that complication of the genitalia, which is so characteristic of the 

 Platyhelminthes . 



In freshwater Tricladida copulation occurs fairly freely among 

 animals kept in glass jars, where they are easily observed. When the 

 penis is retracted its lumen is closed so that sperm cannot escape 

 into the genital atrium, whence it might find its way up the oviduct 

 (Fig. 149). When the penis is thrust out through the genital opening 

 during copulation it is dilated on extrusion, so that the lumen is 

 opened. This dilation also causes the penis to fill completely the 

 genital atrium and opening, so that the opening of the oviduct into 

 the genital atrium is blocked and no sperm can enter or ova escape. 



