PHYLUM PLATHELMINTHES 113 



usually located at the posterior extremity and consists of varied 

 forms of sucking discs and combinations of hooks and spines.' 

 These hermaphroditic individuals produce eggs which undergo 

 direct development without complicated larval changes and 

 without alternation of hosts. The young parasites may immedi- 

 ately attach themselves to the body of the host which sheltered 

 the parent fluke. 



Two orders are recognized on the basis of structure of the 

 posterior organ of fixation. In members of the order Mono- 

 pisthocotylea (Gyrodactylus (Fig. 63 A), Dactylogyrus, Ancyro- 

 cephalus, Nitzschia), this posterior organ is a single structure 

 provided with extremely varied combinations of hook and spines 

 for attachment to the skin or gills of fishes. In the Polyopistho- 

 cotylea (Alicrocotyle, Polystoma, Diplobothrium, etc.), each 

 individual bears two or more posterior suckers supplemented by 

 hooks or spines. Skin and gills of fishes and body surface, 

 urinary bladder, and pharynx of amphibians and reptiles are the 

 chief seats of infestation by representatives of this second order. 



Subclass DiGENEA 



Almost all of the Digenea are internal parasites which require 

 at least two hosts for completion of the life cycle. Instead of the 

 direct development characteristic of the monogenetic trematodes, 

 the digenetic forms always pass one generation or more in a 

 mollusc before the stage capable of living inside the final host is 

 attained. There is thus an alternation of generations accom- 

 panying an alternation of hosts. ]\Ian and his domestic animals 

 act as hosts for numerous species of the digenetic trematodes, 

 hence they hold much of interest for the student of medical 

 zoology. 



Digenetic trematodes are provided with one or two suckers 

 the anterior of which usually surrounds the mouth opening. 

 Conspicuous chitinous hooks, so common in the Monogenea, are 

 lacking in the Digenea, though relatively smaller spines are of 

 common occurrence. The ventral sucker, or acetabulum, is 

 usually near the middle of the body, though in some instances 

 it is entirely wanting (monostomes). From the mouth the 

 digestive system continues backward as a single tube which 

 divides to form two lateral branches, the ceca, for digestion and 

 distribution of the food. Some forms seem to depend entirely 

 upon absorbing their nutriment from the host directly through 



