Thus, summarizing what has been said before, we can see 

 that within the limits of both groups of monogenetic trematodes charac- 

 terized by different types of larvae, basically there are three (or four) 

 main directions of evolution of the attaching organ. If one should analyze p. 332 

 again the changes in other systems of organs, examining them within 

 the limits of each of the groups cited, he will see that a number of 

 peculiarities in all groups changes quite normally and 

 similarly within the limits of each of the groups. Thus, the digestive 

 system becomes more complex and increases in relative size by way 

 of the growth of the lateral dlvertlculae and of internal commisures in the 

 direction from the more simple, among the species (of smaller sizes) 

 with more primitive armature, to more complex among species (more 

 often of larger sizes) with a complex system of attaching organs. At 

 the same time, however, this tendency cannot be attributed to the in- 

 crease in the size of the body because in a number of cases the species 

 with the more complex attaching apparatus have also a more well- 

 developed digestive system even though they have the same body sizes. 

 Thus, for instance, the representatives of the genus Ancyrocotyle 

 have a more simple attaching armature and a simple unbranching intes- 

 tinal tract whereas Megalocotyle or Macrophyllida, which are approxi- 

 mately of the same size, possess a more complex armature and intestinal 

 trunks with numerous lateral outgrowths. Within the limits of all groups 

 the attachment of the anterior end takes place by means of the secretions 

 of the cephalic glands among species with the more primitive attaching 

 apparatus, and, during further complication, the change in the beginning 

 proceeds toward the direction of the concentration of the glands and 

 then their replacement by nnuscular formations of the sucker-type. It 

 is understandable that there is no absolute convergency but the parallelism 

 of the tendency is very distinct. Thus, within the limits of the first 

 direction of evolution the process reaches maximal concentration of 

 cephalic glands, in the second direction- -to the formation of two 

 cephalic suckers and in the third- -one sucker and in the fourth- -the 

 appearance of two suckers inside the buccal cavity. 



Because of the fact that the excretory system is poorly 

 studied it is diffictilt to speak about its changes, however, its complication 

 apparently develops concurrently withthe complication of the digestive system 

 and with the general growth of the dinnensions of the body. The changes 

 in the nervous system bear the character of its greater concentration and, 

 as has already been indicated (see pages 50 and 327 ), the increase in the 

 development of ganglia in the posterior part of the body in propoi-tion 

 to the complication of the attaching armature. The given process is 

 more sharply expressed in the first three lines, in connection with the 

 peculiarities of the development of the attaching apparatus indicated 

 above for the first trunk in comparison with the second (see page 328 ). 



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