more or less simple effering ducts, consisting most probably of a 

 single obtype opening exteriorly by an independent (as regards the male 

 sex system) aperture. 



Such are the basic traits of the primary nnonogenetic tre- 

 matodes reconstructed on the basis of the data of the structure of con- 

 temporary larval and adult monogenetic trematodes. If we compare 

 this hypothetical form with contemporary Turbellaria we shall see its 

 unusual similarity with a number of Rhabdocoela--Dalyellidae. 



As was already indicated (page 291) apparently the primary 

 forms had a wide specificity and were encountered on the body surfaces 

 of their hosts. At the same time their life cycle was connpletely 

 primitive, i.e., the differences between the structure of the larva 

 and the adult animal were minimal. Maturity was reached in a very 

 short period and the sex system functioned either during the entire 

 year or in its warmest period, i.e. , it was limited mainly by the 

 temperature reginne and not by the life cycle of the host. ^ 



_ 



Hence the above-mentioned supposition about the presence of only 

 the obtype, for the absence of the uterus is characteristic for forms 

 with a continuous or extended period of egg -laying and not the 

 apportioned one. 



Further biological process of the development of the group 

 took place by way of greater narrowing of the degree of specificity and 

 the greater adaptation to parasitizing special sections of the body of the 

 host and of timing to the life cycle of the host, i.e. , the concentration 

 of reproduction with a certain period and of the working out of corre- 

 sponding types of morphophysiological structures and peculiarities. 

 However, this progress did not proceed in one general direction but 

 several, reflecting the specificity of different peculiarities of the 

 morphology of the groups of hosts, the direction of the joint existence 

 of a given pair of species of parasite and host and those concrete con- 

 ditions of the medium in which the host lived. The latter are reflected 

 in its biology, through it- -on the parasite and they also influence the 

 latter directly. 



At the same time one must bear in mind the eight variants 

 of possible historical interrelations of the species of the parasite 

 and the host, about which we have already spoken (see page 298). 



381 



