93 J LIFE HISTOR Y OF TREMA TODES~FA UST 93 



hand, no Laurer's canal has been found in the groups where no canal is present 

 in the adult. Students of cercariae have not as a rule recorded the canal 

 in trematode larvae. Ssinitzin (1905, Figs. 62, 67, 74) has found it in the three 

 xiphidiocercariae, Cercaria gibba de Fil., C. prima, and C. secunda. The 

 constant development of this organ in the several groups described in this 

 paper shows that the canal originally had an important place in the genital 

 processes of the Digenea. 



The pronounced difference in structure between parthenogenetic ova which 

 develop into rediae and those which develop into cercariae has been discussed 

 (p. 18). This difference has been found to bear no relation to the phenomenon 

 of maturation, since the chromosome count in the mature cells is the same, 

 whether redia or cercaria is to be produced. In fact, it seems probable that 

 the differentiation occurs before maturation. It is significant that the ova 

 which develop into redia are comparatively simple while the ova which develop 

 into cercariae are extremely complex. In this connection it has been observed 

 that embryos developing from germ-balls produced from ova free in the 

 body caxaty produce daughter rediae. The maturing ova from the germinal 

 epithelium lodged in the body wall of the parthenita develop either into rediae 

 or cercariae. 



In his experiments on planarians Child (1915) has shown that starvation 

 and fragmentation (fission) secure a rejuvenescence for the individual. The 

 less differentiated individual is on the whole the younger one. From the 

 present study on the life-history of trematodes there is justification for the 

 behef that the undifferentiated eggs produce daughter parthenitae because 

 they are simple, i.e., younger, while the more highly differentiated eggs grow 

 into cercariae because they are physiologically old. Interpreted in this light, 

 the parthenitic individuals of the Digenea are physiologically younger than the 

 cercariae and the adult hermaphroditic forms because their structure is 

 simpler. They have sacrificed complexity of structure to meet the needs of 

 the parasitic life, and in so doing have become remarkably rejuvenated. In 

 two species, Cercaria diaphana and C. micropharynx extreme simpHcity has 

 been assumed in the sporocyst, for the germ-balls develop from any cell of 

 the body wall. 



The writer believes that the ability of the parthenita to reproduce daughter 

 rediae or sporocysts for two or more generations rests on the simplicity of the 

 ovum and the relative simpKcity of the parthenita, especially as regards the 

 undifferentiated mesoderm cells. If this rejuvenation can be continued 

 indefinitely, the parthenogenetic generations can also continue indefinitely 

 without the intercalation of the hermaphroditic cycle. 



Child (1915:407) has stated that "in many cases parthenogenetic eggs 

 are apparently less highly difi'erentiated morphologically, and younger phy- 

 siologically, than zygogenic eggs of the same species." The present study 

 makes it necessary to add that in cases where the parthenogenetic eggs may 

 develop into parthenitae or cercariae, the eggs which develop into the former 



