74 ILLINOIS BIOLOGICAL MONOGRAPHS [292 



In view of the facts stated above the writer believes that an accurate 

 knowledge of the more fundamental systems of organs will reveal group 

 relationships which have heretofore passed unnoticed. In the present 

 systematic groups homology of organs is a factor which has been generally 

 passed over. This can best be demonstrated by a careful study of the 

 developing organs in the early stages of the life history of the individual. 

 Up to this time little has been done on the life histories of the Monosto- 

 mata. Von Siebold (1835) and Van Beneden (1861) studied the early 

 stages of the parthenogenetic phase. La Valette St. George (1855) de- 

 scribed under the name of Monostomum flavum a cercaria which he believed 

 to belong to this species. From his descriptions and figures one can see 

 diagnostic points which tend to show that this cercaria belongs to the 

 Notocotylidae on the basis of the well developed oral sucker, the posses- 

 sion of the locomoter pockets described by Looss, Cort, and Faust for 

 Notocotylid species. The fact that La Valette St. George showed the 

 intestine anastomosed posteriorly does not furnish evidence to the contrary 

 since the crura of the Notocotylid approach each other in the posterior 

 end of the worm and this may easily be mistaken for anastomosis or on the 

 other hand the author may have misinterpreted the excretory ducts for 

 the crura of the intestine. The characteristic features are the absence of 

 the pharynx and the presence of the three eyespots which appear to be 

 characteristic of the Notocotylidae. In addition to these Haldemann, 

 Leidy, Cort and Faust have described some six or seven monostome 

 cercaria from American hosts all of which have been referred to the Noto- 

 cotylidae. While the writer has had opportunity to study immiature 

 stages of N otocotylus urbanensis, the preserved material has yielded only 

 few facts that can be interpreted as of phylogenetic importance. These 

 will be discussed in a subsequent section of this paper. As yet no Mono- 

 stome life history has been demonstrated experimentally and the develop- 

 ment of the important systems of organs has not been followed in the life 

 history of even a single species. This seems to the writer to be a necessary 

 step to be followed out in the major families in order to demonstrate the 

 phylogenetic relationships of the large groups. 



INTERRELATIONSHIP OF THE MONOSTOME FAMILIES 



Before entering into the discussion of the probable origin of the Mono- 

 stomes it seems fitting to discuss the interrelationship of the families as a 

 unit or natural group of Trematodes. A comparison of the family diagno- 

 ses, given earlier in this paper, shows a striking contrast in each of the 

 families discussed and of the families not included the same striking con- 

 trast may be drawn. No system of organs is the same in all of the families 

 save perhaps the nervous system, so far as it has been made out, which is 

 essentially the same in all the trematodes. The excretory system differs 



