Basically the larvae of all monogenetic trematodes are charac- 

 terized by the following signs; first, the presence of a ciliated covering, 

 located mainly in three zones --near the head end, in the middle of the body, 

 and near the attaching disc; second, by the presence of head glands the 

 efferent ducts of which are grouped by bunches opening outside in the 

 anterior head lobe of the body; third, the presence of strongly differentiated 

 systems of internal organs --of the digestive system with the pharynx and 

 intestine developed to a certain degree, with the excretory system with 

 basic ducts and a number of protonephridial cells and a nervous system 

 with isolated head ganglia and longitudinal nerve trunks and one to two or 

 four eyes (a number of exceptions); fourth, powerfully developed attaching 

 discs always equipped with chitinous armature consisting of a certain 

 number of varying or similar hooks. 



One can expect that the enumerated characters probably were 

 also characteristic for the primary larvae of monogenetic trematodes and 

 it appears to us that in addition, one should consider the location of the 

 buccal aperture on the ventral surface much closer to the middle of the 

 length of the body than is observed among contemporary species, as very probable, 

 for we see that the buccal operture is located closer to the midole of the body 

 among larvae of more primitive contemporary Monogenoidea than among the 

 more highly organized ones. The nature of the ciliary covering of contempo- 

 rary species does not allow us to speak with certainty concerning the direction 

 of its evolution, but nevertheless it is probable that initial forms possessed 

 a continuous ciliary covering and its division into separate zones is a 

 secondary phenomenon. Further, it is completely clear that hooks were 

 the initial armature of the ancestors of monogenetic trematodes and not 

 suckers or clamps which, just as in the individual development, histori- 

 cally appeared later. In all probability among the ancestral species the 

 chitinous armature of the attaching disc was represented by a great nunnber 

 of hooks equal in form and size and lying along the edges of the posterior 

 end of the body which was not yet differentiated into the fornn of a disc 

 (which was, nobis ) apparently already somewhat flattened, as we observe 

 among a number of Rhabdocoela. A clearly expressed tendency toward the 

 decrease in the number of edge hooks in proportion to their morphological 

 complication among contennporary monogenetic trematodes serves as a 

 basis for these suppositions. Such a larva (Fig. 114) of monogenetic trema- 

 todes does not differ in principal from the adult form which develops from 

 it. Basically the differences can be reduced to the appearance and develop- 

 ment of the sex system and the progressive growth of the organs of all 

 systems. Concerning the sex system, we can maintain that it was incepted p. 97 

 and developed in the posterior half of the body behind the end of the intestinal 

 system. Finally, it is not less probable that the ciliary covering of the 

 larvae remained during their attachnnent to the host in the beginning as this 

 happens in a number of ectoparasitic Turbellaria. Having thus recreated 

 in considerable measure a promonogenetic trematode (Fig. 115), we see 

 that its similarity with the Turbellaria is so great that we could, without any 



98 



