FLATWORMS 



genital atrium. The genital pore opens on the ventral side of the body, 

 posterior to the mouth, and connects the genital atrium to the exterior. The 

 relationship of the penis to its cavity, and to the genital atrium, is similar 

 to that of the pharynx to its sheath. During copulation, the penis is thrust 

 from the genital pore in the same manner as the pharynx is thrust from the 

 mouth during feeding. 



The female system consists of a pair of ovaries, from which the ova arise, 

 located usually just behind the head region. A single oviduct leads posteriorly 

 from each ovary, receiving in its course a series of smaller ducts from the 

 numerous yolk glands which surround it throughout its length. The two 

 oviducts may unite posteriorly at the midline to form a common chamber, the 

 female antrum, lying just behind the genital atrium and connected to it. In 

 other species the paired oviducts join the genital atrium directly. Most plan- 

 arians possess also a bulbous, blindly ending chamber, the copulatory bursa, 

 opening either from the genital atrium or from the female antrum and extend- 

 ing a short distance dorsally and anteriorly. 



The functional aspects of this complex system may now be considered. 

 Se.xual reproduction in planarians usually involves copulation, in which the 

 genital pores of two worms are brought together and the penis of each worm 

 is inserted through the genital opening and into the copulatory bursa of the 

 other. Mutual exchange of spermatozoa from the spermiducal vesicles thus 

 occurs, and copulation is completed by this insemination. After the partners 

 have separated, the spermatozoa received in copulation are transferred, or 

 move, from the copulatory bursa into the paired oviducts and finally come to 

 rest in the anterior end of each oviduct. Here, adjacent to the ovary and 

 separated from it by a membrane, is a seminal receptacle, in which sperma- 

 tozoa are stored. Eggs released by the ovary break through the membrane, 

 are fertilized, and pass down the oviduct where they are surrounded by yolk 



Fig. 1 1 .7. Planarian egg cap- 

 sules, or cocoons, and juvenile 

 planarians emerging. (Drawn 

 by George T. Kline.) 



321 



