TYPE PLATYUELMINTHES. 147 



Two orders may be recognized as occurring in the Tre- 

 nt) atoda. 



1. Order Polystomeae. 



The Polystomese are for the most part ectoparasites and 

 present fewer signs of degeneration than do the endopara- 

 sitic members of the class. The apparatus for adhering to 

 their hosts is usually strongly developed, several suckers 

 usually being present, as, for instance, three in Tristomum 

 and seven in Polystomum integerrimum (the latter parasitic in 

 the urinary bladder of the Frog), and in addition a number of 

 chitinous hooks may occur, as in Gyrodadylus and Sphyranura 

 (the latter parasitic on the skin of Menobranchus). In accord- 

 ance, too, with their mode of life, sense-organs in the form of 

 eyes and probably of tactile papillae on the skin occur, and 

 furthermore the processes of development are much simpler 

 than in the endoparasites, as will be seen later. 



Some peculiar anomalies occur in the life-histories of some of the Poly- 

 stomeae, as, for instance, in the GyrodaetyliLS, which lives upon the gills of 

 the Carp. It is a viviparous form, and the young while still within the 

 body of the parent may already have become mature and contain young 

 likewise, which again may contain ova in course of development, four gen- 

 erations being thus enclosed one within the other. Diplozoon, which lives 

 likewise on the gills of Cypriuoid fishes, is peculiar in that at the time of 

 sexual maturity two individuals become fused with one another in the 

 form of an X, the fertilized ova giving rise to a single form formerly 

 known as Diporpa. 



2. Order Distomeae. 



This order includes endoparasites which show a more 

 marked degeneration than do the members of the preceding 

 order. Eyes may be present in the young but are absent 

 in the adult, and furthermore a very complicated metamor- 

 phosis is passed through in the development. The suckers 

 for adhesion to the host are either one (Monostomwti} or 

 two (Distomum), and as a rule no chitinous hooks are pres- 

 ent. 



Among the more interesting members of this order are Distomum 

 hepaticum, a large form measuring 2-3 cm. in length and inhabiting the 



