close to one proposed by us in our work in 1937 and we fully agree with it. 

 If one is to attempt to show on this diagram not only the correlations be- 

 tween the separate groups but also the degree of their divergency it will 

 acquire the following aspects (Fig. 315) and will be understood without 

 explanations . 



The final question on which we would like to pause is the 

 problem of the correlations of peculiarities of the biology of Monogenoidea 

 and Cestoidea. We were told repeatedly about the circumstance that it is 

 not understandable by what means this truly ectoparasitic group, which 

 monogenetic trematodes represent, gives rise to such typical intestinal 

 parasites as Gyrocotyloidea and tapeworms. It seems to us that it is 

 possible to find sufficiently convincing, although to a certain extent con- 

 jectural answers to this question. Generally among monogenetic trematodes 

 the transition to endoparasitism is far from being so rare as is usually 

 pictured. Present representatives of the genus Acolpenteron are endo- 

 parasites parasitizing the ureters of their hosts. According to the studies 

 of Ruszkowski (Ruszkowski, 1931)^ Amphibdella torpedinis Chatin has an 

 obvious tendency towards the transition, be it partial, toward parasitizing 

 the blood system of electric skates. The genus Dictyocotyle parasitizes the 

 body cavity of skates (Nybelin, 1941). Numerous species of Calicotyle live 

 near the cloacal opening of skates, appearing rather more as endoparasites 

 than ectoparasites. A number of Polystomatidae parasitize the urinary 

 bladder of Amphibia and Reptilia, etc. However, it is not these 

 cases that present an interest from the point of view of the transition of 

 ectoparasitism to parasitism in the intestinal cavity. This process it seems 

 to us is connected with the transition of the very numerous monogenetic 

 trematodes towards parasitizing from the gills to the surface of the buccal 

 cavity of their hosts with subsequent advance to the walls of the pharynx 

 and to the anterior part of the esophagus. Inasmuch as we can be sure that 

 this process of transfer from ecto- to endoparasitism took place in the case 

 which interests us among fishes and most probably in Selachii, one can 

 suppose that the degree of differentiation of the separate parts of the intestines 

 of the host could not serve as an important barrier for further advance of the 

 worms from the esophagus to the intestinal tract. The fact that this is so 

 is substantiated by the fact that among contemporary Selachii a number of 

 species of tapeworms parasitize the entire length of the intestines --from 

 the esophagus to its utmost posterior part without preference for any 

 specific part. Thus, we have personally observed similar distribution along 

 the intestinal tract in a number of Tetrarhynchidae. 



As regards monogenetic trematodes, their tendency to transfer 

 to parasitizing a cavity of the body and the anterior part of the intestines is 

 encountered in Capsalidae, Monocotylidae, Polystonnatidae, Hexostomatidae, 

 and Dicliphoridae and others, 1. e. , among a very great number of morpho- 

 logically varying forms which are related to both subclasses. As we have 

 already written earlier, it is not possible to visualize the process of separation 



571 



