As regards the quantitative relations, the gradual decrease 

 from the initially considerable number to the eight, seven, six, and five 

 pairs of hooks encountered among contemporary species is characteristic 

 for the edge hooks. At the same time the reverse process is charac- 

 teristic for the middle hooks --increase of their number from one to two 

 orthree pairs. However, one must stipulate that among the contempo- 

 rary forms the absence of middle hooks is a predominantly secondary 

 phenomenon (see Bychowsky and Gussew, 1955 and page 347). 



One can consider that in the general branch of monogenetic 

 trematodes which have the larvae of the first type, there are different 

 smaller directions of evolution which differ in the nature and direction 

 of the evolution of the attaching apparatus. As the most prinnitive, but 

 at the present time represented by a large number of species, we can 

 consider the direction in which the attachment of the animals to the 

 body of the host takes place only by means of the chitinous armature in 

 all phases of the life cycle (except of course the free-swimming). The 

 second direction of evolution within the limits of the first branch of 

 monogenetic trematodes can be considered the way of adaptation for the 

 attachment to the body of the host during the gradual and historically 

 functional replacement of the chitinous armature directly by the p. 330 



attaching disc itself during the life cycle. At the same time, during 

 the early stages of the life cycle, and also the more historically ancient 

 species (and among the contemporary morphologically more 



primitive), the worms attach themselves by means of the same chitinous 

 apparatus (which is subordinated, in the beginning, to the same morpho- 

 logical principles) and then in later stages the attaching disc .changing 

 more and more, transfers into a more or less powerful sucker, 

 completely assuming the function of attachment. The increase of the 

 attaching power of the disc -sucker is reached by two ways which often 

 proceed in parallel fashion and at the same time; namely, by means of 

 a greater and greater development of its musculature (and at the same 

 time the musculature of the suckers is not smooth at all, but transversly 

 striated- -the functional meaning of this is not fully clear considering 

 the nature of the action of the attaching organs of the present group)^ 

 or (and) by way of increasing the power of the sucker by the formation 

 in it of a number of partitions --septa, which increase the volume of 

 the sucking surfaces. As regards the chitinous hooked apparatus, 

 among more highly organized forms it loses its functional significance 

 during the very early stages of development almost completely, whereas 

 among separate forms the middle hooks begin early to degenerate and 

 perhaps even disappear partially or completely. 



As a similar direction of evolution connected with the way 

 indicated is the adaptation toward the attachment by means of suckers 



386 



