of the attaching disc and its parts the nervous system also forms a 

 more powerful nerve ring in it with strongly developed ganglia which 

 are almost equal in sizes to the cephalic ones. Further the flattening 

 of the body and its relative increase leads to the powerful development 

 of the digestive system, forming numerous lateral outgrowths and many 

 different anastomoses between the trunks. Finally, the sex system also 

 undergoes certain changes, particularly in many species the aperture of 

 the uterus begins to be displaced towards the side of the body which 

 facilitates egg laying independently of the position of the body of the 

 worm. 



In a majority of the cases the animals which became adapted 

 to existence directly on the gills retain their cylindrical bodies and 



the development of the attaching organs proceeds by way of the increase 

 of the chitinous armature mainly in the form of the increase in the 

 number and complication of the separate parts of the attaching apparatus. 

 This, in the final analysis, leads to the motionless form of life in spite 

 of the fact that either of the parts of the attaching apparatus can by 

 itself attach or detach from the body of the host. In connection with 

 the sedentary way of life the anterior part of the body does not need 

 powerful attaching fornriations, but, inasmuch as fixation of the anterior 

 end during feeding is indispensable (see page 81 ), special sucker-shaped 

 fornnations also develop in the species of this line of evolution, forma- 

 tions which often lie directly in the buccal cavity. The increase in the 

 dimensions of the body leads, in parallel fashion to what we saw in the 

 preceding line, toward the same changes in the structure of the intes- 

 tines, that is to an increase in the number of outgrowths extending into 

 all parts of the animal. As regards the sex system,its changes already 

 bear a more special nature, even though it is related to the peculiarities 

 of the attachment of the animals, but without any definite tendency. 



What has been said above about the two basic directions of 

 the evolutionary process among monogenetic trematodes gives us a basis 

 for a casual analysis of the morphophysiological changes taking place in 

 concrete phylogenetic stages and makes it possible for us to evaluate 

 the significance of changes of different morphological characteristics. 



As is evident in the evaluations of ways of evolution we p. 328 



attach special significance to the method of attachment of monogenetic 

 trematodes to their hosts, considering it to be a leading adaptation 

 determining the process of the evolution of the group. However, it 

 does not mean at all that one can utilize only these characteristics for 

 the classification of the phylogeny of the group but, just as we attempted 

 to note and as we shall show further on, the concrete material of the 

 systems of attaching formations undoubtedly gives us the possibility for 



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