472 STANLEY C. BALL 



P. gemellipara from P. cardii in which the shell glands open 

 along the entire central surface of the antrum. 



Although no one has recorded observations of Graffillidae 

 in the act of copulation, it is inferred that pairing occurs. Dur- 

 ing the process, as described for other Turbellaria, masses of 

 spermatozoa from the seminal vesicle of one animal are intro- 

 duced by means of the penis into the atrium of its mate. There 

 may be a mutual exchange, but certain observations lead to the 

 belief that the younger specimens in which the male organs 

 are at the height of their development fertilize the older ones 

 having the functional female apparatus, while older ones do 

 not fertilize the yomiger. 



In this regard it is to be noted that during early life the ani- 

 mals have widely distended seminal vesicles; in the old worms 

 this organ contains only small masses of spermatozoa. More- 

 over the younger specimens in which the ovaries are not func- 

 tional have not been observed to contain sperm in the atrium 

 and antrum. 



Embryos 



In a worm which is rapidly reproducing one finds distributed 

 through the parenchyma capsules (fig. 1) containing embryos 

 in various stages of development, from the newly-fertilized 

 egg to ciliated young, actively moving about in their cramped 

 quarters. During early development the two embryos of each 

 capsule are usually separated by a considerable thickness of 

 yolk, but may in some cases lie so close together that the blasto- 

 meres of one touch those of its mate. The capsular membrane, 

 heavy and well defined while the embryos are small, becomes 

 gradually thinner on account of water absorption and consequent 

 stretching until, by the time the young develop cilia, it is ex- 

 tremely delicate and apt to be ruptured by vigorous pressure 

 of the surrounding organs. When fully enough developed the 

 young break forth from their capsules, swim freely about in 

 the mesenchyme of the mother for a time, and ultimately, as 

 her body undergoes degeneration, slip through the body-wall 

 to the outside. Frequently one is found in the mother's in- 

 testine. 



