The gill cavity of the fishes can indisputably be considered 

 as the initial (and basic or chief, nobis) place of habitation of mono- 

 genetic trematodes. ^ This is fully understandable, because with the 



3 



This does not contradict the location on the surface of the body 

 indicated above for monogenetic trematodes. 



relatively weak attaching capabilities of the primary (ancestral, nobis ) 

 forms the possibility of survival was greater in the given place where 

 the fortuitous disengagenaent of the animal does not entail certain loss 

 of connection with the host, and the chances for secondary attachment 

 are considerable. However, the conditions of the gill cavity are not 

 homogeneous and we see that there are tw^o basic directions of adap- 

 tations to existence in it; the first is the working out of peculiarities 

 which pernnit parasitizing the internal surfaces of the cavity, and the 

 second, parasitizing directly on the very gills, mainly the gill filaments. 

 Although there are considerable differences between both places of 

 habitation, the initial stages of adaptation to both apparently were 

 similar and the divergence in the w^orking out of the morphological 

 peculiarities took place much later. One can note one more charac- p. 327 



teristic peculiarity apparently connected with the conditions of feeding 

 on both basic places of habitation, namely, that in parasitizing on the 

 flat surfaces (interior surface of the gill cavity) the worms retain 

 the ability to move, whereas on the gill filaments they practically lose 

 it very quickly. 



In connection with this, we see that in the adaptation to 

 parasitizing on the flat surfaces, the body of the animals gradually 

 changes from the elongated, terete shape to the flattened, leaf shape. 

 At the same time, the change in the organs of attachment takes place 

 by way of the gradual functional replacement of chitinous formations 

 by muscular suckers which impart greater possibilities of increasing 

 the ability of the worm to attach while maintaining its ability for 

 quick disengagement. The peculiarities of feeding of the forms of this 

 line of evolution of monogenetic trematodes apparently demands the 

 moving of the body of the animal during feeding and, because of the fact 

 that the animals move like leeches, greater or more powerful attaching 

 fornnations appear on the anterior end of the body, but again changing 

 in a definite direction from the glandular border toward the glandular 

 sucker type and then to the purely suction organs of the type of the 

 suckers of digenetic trematodes. As regards the internal organization 

 of the worms, here also takes place a number of changes connected 

 with the same way of adaptation. Thus, with the greater development 



382 



