82 ILLINOIS BIOLOGICAL MONOGRAPHS [362 



the Eastern hemisphere disagree with each other and agree with forms 

 parasitic in the urinary bladder and oral cavity of North American 

 turtles. The turtle parasites have a very similar structure, whether 

 parasitic in the urinary bladder or in the pharyngeal cavity. Further- 

 more, if the observations of Zeller are correct and the individuals of 

 P. integerrimum becoming mature on the gills of tadpoles lack external 

 vaginae and have a spherical testis and a single egg in the uterus, one is 

 entirely at a loss to explain the variation existing in the genus. 



In the Aspidogastridae the young individuals have an oral sucker 

 and a small posterior acetabulum without dividing ridges, and very 

 closely resemble young distomes. The mode of infection is almost en- 

 tirely unknown, and this offers a promising field for investigation. The 

 discovery of the sexual form of Stichocotyle by Odhner (1898) estab- 

 lishes the fact that at least one species of the Aspidogastridae has an 

 intermediate host. Nickerson (1895) observes, "Owing to the well 

 known tendency of fresh water conditions to obliterate larval life, it may 

 well be that Aspidogaster has secondarily lost a more or less compli- 

 cated series of changes, which have been retained by its relatives inhabit- 

 ing salt water." The presence within the family of both direct and 

 indirect development, together with other characters common to both 

 the Heterocotylea and Malacocotylea designate it as an intermediate 

 group. The morphological structure is similar to that of the Mala- 

 cocotylea while the manner of development is similar to that of the 

 Heterocotylea. Whether the Aspidogastridae are primitive forms or 

 are secondarily degenerate is as yet undecided. The simple and archaic 

 character of the intestine, the eye spots, the direct development and the 

 ectoparasitic habit as it occurs in the family, together with the para- 

 sitic infection of molluscs by adult forms strongly suggests a very primi- 

 tive and ancient group. It is probable that complete evidence concern- 

 ing the structure and life history of this family would go a long way 

 toward solving the problem of whether the invertebrate or the vertebrate 

 is the original host and the attendant problem of the origin of double 

 hosts. 



The Paramphistomidae appear to be a primitive family of the 

 Malacocotylea that have retained the original caudal sucker, altho 

 certain species show specializations of the organ from the simple spher- 

 ical type. Considerable light is thrown on the relationships of the 

 Malacocotylea by the recent work of Odhner on a natural system for the 

 digenetic trematodes. He strongly advocates the view that the mono- 

 stomes are a group which have no family entity, and consist of individual 

 forms derived from various distome groups which have alike lost the 

 acetabulum. Pointing out close and fundamental agreement in internal 



