308 ILLINOIS BIOLOGICAL MONOGRAPHS [308 



The resemblance of the heads and suckers of Proteocephalids to 

 those of Cyclophyllideans is, however, more apparent than real. The 

 heads of the latter group are not readily mobile. Except in those cases 

 where a protrusible rostellum is present the heads do not change shape 

 much. The suckers, moreover, are strongly cupped at all times and 

 are not much moved by the muscles of the head. The heads of the 

 Proteocephadids are, as the name implies, ever changing. They undergo 

 a multitude of changes of form and appearance. The suckers may be 

 strongly cupped when contracted or the cavity may be entirely obliter- 

 ated by the stretching of the sucker. The suckers are very mobile, 

 being capable of being pushed out, forward or backward or laterally, 

 and retracted. This movement of the suckers adds greatly to the protean 

 character of the head. The whole anterior part of the head may in some 

 species be retracted within the swollen bulbous basal part of the head. 

 In respect to the movement of the head and the action of the suckers 

 the Proteocephalids resemble the other Tetraphyllideans. 



Another point of resemblance between the Proteocephalids and the 

 other Tetraphyllideans is the presence of a degenerate fifth sucker in 

 many species of the two groups. In the Cyclophyllidea a somewhat 

 similar appearing structure is to be found in certain species. Here it 

 is probably a degenerate rostellum and not a degenerate fifth sucker. 

 Structurally the Proteocephalidae are to be considered as being closely 

 allied to the Tetraphyllidea while their relationship to the Cyclophyl- 

 lidea is distant. The relation of the Proteocephalidae to the other 

 groups of cestodes is remote. The members of the order Tetraphyllidea 

 other than the Proteocephalidae and Monticellidae are characteristic 

 parasites of marine fishes, mostly selachians. The freshwater fishes have 

 arisen from the marine fishes and their parasites have doubtless arisen 

 from the parasites of the marine fishes, the Proteocephalids coming from 

 a Tetraphyllidean ancestry. 



Just which fish species was responsible for the bringing of this 

 Proteocephalid stock into fresh water is a matter of conjecture. Since 

 this group of parasites is found to be parasitic in members of so many 

 families of freshwater fish it may be inferred that some of the older 

 and more primitive Teleostomi, a Ganoid, brought into the freshwater 

 environment a Tetraphyllidean with a simple type of head and suckers, 

 altho this simplicity may have been a later evolution. Another hypothe- 

 sis is that these parasites were introduced into fresh water by representa- 

 tives of several families of fish such as the Salmonidae, the Gadidae, or 

 the Anguillidae. Among the Salmonidae certain species have acquired 

 the permanent habit of remaining in fresh water while others enter 

 fresh water only to breed. The species of Anguilla go back to salt water 



