80 ILLINOIS BIOLOGICAL MONOGRAPHS [360 



RELATION OF THE FAMILIES TO THE ORDER 



The trematodes are generally regarded as descended from a tur- 

 bellarian-like ancestor which possessed a posterior sucker. With the 

 assumption of the parasitic habit adaptations began in various directions. 

 The ectoparasitic forms retained many of their former characters while 

 the added protection and food supply afforded those specializing toward 

 endoparasitic existence provided for perpetuation and distribution of 

 the species thru the excessive development of the reproductive apparatus. 

 The development of the ectoparasitic forms is simple and direct while 

 that of most if not all endoparasites has been complicated by the inter- 

 polation of one or even more secondary or intermediate hosts. 



The differences in type of adhesive apparatus may in a general way 

 be explained thru differences in habit. The oral sucker has developed 

 thru continued adhesion by the anterior end in maintaining position, in 

 locomotion, and in securing food. In the Gasterostomidae the mouth is 

 on the ventral surface and an independent anterior sucker is developed, 

 altho this is undoubtedly a secondary feature, as in the cercariae of these 

 forms there is a single anterior oral sucker. In response to the constant 

 necessity for strong adhesion the ectoparasitic species have developed 

 accessory posterior organs of attachment, while in most of the endopara- 

 sitic forms the acetabulum has migrated anteriad or disappeared entirely. 



The general classification of Monticelli, which is followed in this 

 paper, is based primarily on the character of the adhesive apparatus. 

 In the Heterocotylea the posterior sucker has been replaced by a disc 

 which bears suckers and hooks ; in the Aspidocotylea the acetabulum has 

 become specialized into a multiloculate adhesive organ ; and in the Mal- 

 acocotylea the acetabulum may be retained in its primitive terminal posi- 

 tion, or it may have migrated anteriorly, in certain cases being reduced 

 and in others disappearing entirely. In the young individuals of many 

 forms in each of the three groups there is a single posterior sucker and 

 this fact adds weight to the theory that the present groups are descended 

 from a primitive form with a simple posterior sucker. In the young 

 stages of all the Aspidogastridae there is a simple posterior sucker and 

 the worm closely resembles a young distome. In the early stages of the 

 Heterocotylea the reversion to the ancestral conditions is not so complete, 

 and specialization in this group shows clearly that it is widely separated 



