86 ILLINOIS BIOLOGICAL MONOGRAPHS [86 



PROBLEMS PRESENTED 



INTERRELATION OF TREMATODES 



In working out new species of organisms, especially from new geographi- 

 cal areas, there are often physiological and morphological facts that are 

 of general significance in the light of previous studies. Relationship is the 

 ultimate problem for all workers in morphology. While light is shed on 

 phylogeny by the discovery of new species, it is futile to expect to found a 

 system of phylogeny on a single species. With these limitations in mind the 

 writer presents some of the more important questions that arise from the 

 study of the trematodes of the moUusks of the Bitter Root Valley. 



In the early days cercariae and rediae were considered as different groups 

 of the animal kingdom, and it was not until the life-history studies of Leuckart, 

 La Valette (1855), and Ercolani (1881, 1882) had been published that the 

 genetic relationships of cercariae and parthenitae were established. The 

 germ layers of all generations of trematodes have a similar origin, and impor- 

 tant structures of the group present the same problem. 



Certain investigators of recent time have come to regard the trematodes 

 as a pol>'phyletic group. Their conclusion has resulted, perhaps, from their 

 lack of study and consequent inability to recognize the fundamental resem- 

 blance of the genital, excretory and nervous systems of the various sub-divi- 

 sions of the group, especially during the developmental stages. 



From more convincing observations Odhner (1907) has concluded that the 

 Monostomata are polyphyletic. He has noted among certain monostomes a 

 structure comparable to the pharyngeal pockets of amphistomes, and among 

 others a primitive acetabulum (1911). In a study of monostome cercariae 

 from the Bitter Root Valley, the similarity of the nervous system and genital 

 cell masses of Cercaria pelliicida and of those systems in Gastrothylax gre- 

 garius Looss has been investigated. 



Both Cercaria pellucida and Gastrothylax gregarius show paired brain 

 gangha closely set together with only sUght constriction of the dorsal com- 

 missure. In both species the nerve cells lie superficially upon the central 

 nerve ganglion masses. Moreover, relatively large posterior ventral and 

 smaller lateral and dorsal trunks, together with their respective positions, 

 constitute a series of similarities not to be overlooked. The lack of pharyn- 

 gealis, palatinus, and dorsolateral commissure (Figs. 123, 124) serves to 

 show that the nerve complex of the monostome larva Cercaria pellucida is 

 not typically distomate. Its nerve structures are much more readily referred 

 to the Gastrothylax type. 



In the second place certain features of the genitalia of Cercaria pellucida 

 and Gastrothylax suggest a common ancestry. The median ovary in the 

 subcaudal region; the paired testes lateral to the ovary; the dendritic vitel- 

 laria, located in two series, and, finally, the parallel course of the uterus and 



