Finally, the sex system shows a number of parallel changes 

 in all groups. Thus, within the limits of each group, the testes have a 

 tendency toward the increase in number from one to several or to the 

 formation of a large follicular mass about which it is impossible to 

 say whether it represents a single or a multiple organ (see Dogiel, 

 1954a, and also page 57 ). Parallel to this, in the majority of the 

 cases the formation of the seminal reservoirs and generally the 

 complication and elongation of the nnale sex ducts also takes place. 

 The female sex system also has a tendency towards increasing in size 

 and toward complication of the shape of the ovary, and the latter begins 

 more and more to separate sharply into two parts --one containing 

 obgonia and one containing formed oocytes. As a rule the ducts of the 

 fenraale systenn become complicated and the appearance of the ductus genito- 

 intestinalis is very characteristic among more highly organized groups 

 ^s we suppose in parallel fashion and independently in Polystomatidae 

 and Oligonchoinea, see pages 398-402 ). Finally one cannot fail to note 

 that the appearance and development of the uterus also takes place in 

 parallel fashion in all groups which were noted. 



At the same time we see also the changes of the biology of p. 333 

 the animals in the direction of the complication of their life cycle and 

 of its greater and greater adaptation to the life cycle of the host (see 

 page 128 ). It is understandable that numerous morphological changes, 

 and in the first place- -chaiiges of the sex system, are closely connected 

 and depend on the change of the life cycle, reflecting the peculiarities 

 of new physiological adaptations. 



In conclusion, we suppose that in the formulation of the 

 system of monogenetic trematodes reflecting the phylogenetic relations, 

 it is indispensable, first of all, to pay attention to the attaching apparatus 

 as a leading adaptation characterizing a given group, taking into con- 

 sideration the changes in other organ systems. Possible objections and 

 attempts to build the system on the basis of other characteristics are 

 not justifiable and do not reflect true relations within the limits of the 

 group. This is convincingly substantiated by the coincidence of the 

 materials on the directions of development of the larvae and the 

 tendencies of the development of adult monogenetic trematodes •v^ich 

 have just been expressed. From all that has been said it is clear that 

 in the attempt to build a system on the basis of only the characteristics 

 and tendencies in the development of the sex system, let us say, we 

 will obtain only an artificial unification or joining of speciescompletely 

 different in origin which have only a converging sinnilarity (which can 

 be easily proven by the structure of the attaching apparatus). 



390 



