10 ILLINOIS BIOLOGICAL MONOGRAPHS [290 



described P. appendiculatum from Squalus catulus. Dujardin (1845) 

 transferred Diclibothrittm crassicaudatum Leuck. 1835 = Diplobothrium 

 armatum Leuck. 1842 to the genus Pohstoma, and listed as additional 

 species, P. duplkatum, P. pinguicola, P. ocellatum, P. integerrimum, and 

 P. appendiculatum. Diesing (1850) named P. loliginis and P. appendic- 

 ulatum as tj-pes of new genera Solenoeotjle and Onchocotyle. He re- 

 moved P. armatum to the genus Diclibothrium Leuck. and retained in 

 the genus Polystoma only the species P. integerrimum and P. ocellatum. 

 The genus Polystoma together with the genera Tetrastomum, Gry- 

 porhynchus, Hexathyridium, Notocotyle, Aspidocotyle, and Aspidogas- 

 ter were included by the same author in the tribe Polycotylea. 



In his revision Diesing (1859) reduced the trematodes to the rank 

 of a tribe and divided the group into three subtribes: Acotylea, Coty- 

 lophora, and Plectanophora. The second of these subtribes he subdivi- 

 ded into three families : Monocotylea, Tricotylea, and Polycotylea. The 

 last of these corresponds almost identically with his former tribe Poly- 

 cotylea. He rejected Gryporhynchus. and added the genera Ancyroceph- 

 alus, Plagiopeltis, Heptastomum, Onchocotyle, Cyclocotyle, and Solen- 

 oeotyle. In the family Polycotylea he recognized two subfamilies: 

 Aplacocotylea with the suckers set directly in the body, and Placocotylea 

 with the suckers set in a median posterior plate. In the latter he in- 

 cluded the genera Onchocotjde, Polystoma, Cyclocotyle, Aspidocotyle, 

 Aspidcgaster, and Solenocotyle. 



Then followed the great work of van Beneden (1858) with an ex- 

 perimental demonstration of the "direct" development of the many- 

 suckered ectoparasitic trematodes, and the "indirect" development of 

 the distomes. For these two groups he proposed the names Monogenea 

 and Digenea. In the former he recognized two families: the Tristomi- 

 dae with a single posterior sucker, and the Polystomidae with several 

 posterior suckers. In the Polystomidae he included the genera Polystoma, 

 Diplozoon, Octobothrium, Axine, Onchocotyle, Calceostoma, and Gyro- 

 dactylus. 



Later van Beneden and Hesse (1863) made the genera Oetocotyle 

 ( = Octobothrium), Udonella, and Gyrodactjdus types of new families, 

 thus increasing the number of families to five. Many additional genera, 

 both old and recently described, were now for the first time placed with 

 the Monogenea. But in the family Polystomidae these authors retained 

 only two genera, Polystoma and Erpocotyle ; and in the genus Polystoma 

 was listed only a single species, P. integerrimum. 



Taschenberg (1879) reverted to the earlier classification of van 

 Beneden and adopted the division of the monogenetic trematodes into two 



