is either hampered or practically completely impossible. As regards 

 the anterior end, its attachment and disengagement takes place very 

 easily and very quickly for, during feeding the worms usually move their 

 cephalic ends very quickly, attaching them only for a short time during the 

 seizure of the food. 



Thus, the body of the primary monogenetic trematodes was 

 differentiated into functionally and morphologically different sections -- 

 the anterior end which played a role in alimentation and the posterior 

 served for the fixation of the animal to the body of the host. At the 

 same time the posterior end, in addition to the chitinous armature, also 

 retained a number of agglutinating glands which is observed among many 

 contemporary forms. 



As regards the anterior end, its attachment among primary 

 forms took place apparently with the help of unicellular glands located 

 along the anterior end of the body in a more or less wide band analogous 

 to the lobe glands of the Turbellaria. 



The coverings of promonogenetic trematodes most probably 

 were represented by a ciliary epithelium; however, the nature of its 

 distribution is not completely clear. Taking into consideration that a 

 process of reduction of the ciliary covering, mainly by way of its dis- 

 appearance on the dorsal side {often among the oldest ages), is observed 

 among a number of Turbellaria (Beklemishev, 1937) one can suppose that 

 among the original forms of monogenetic trematodes the ciliary epithelium 

 was also rarified on the dorsal side and had the tendency toward the for- p, 325 

 mation of separate ciliary zones. At any rate it is quite probable that 

 the contemporary, very specialized covering of monogenetic trematodes 

 appeared gradually and did not exist am.ong the original forms. 



The internal organization of promonogenetic trematodes can 

 be characterized fairly accurately. The digestive system was represented 

 by a buccal opening lying near the middle of the body, by a sufficiently 

 powerful pharynx of the type pharynx-plicatus and by a sac -shaped in- 

 testine. Digestion took place by means of phagocytosis similar to that 

 which takes place among the majority of Turbellaria and all the lowest 

 Monogenoidea. The excretory system apparently did not differ in any 

 way from the one of the contemporary forms with the exception perhaps 

 of its greater simplicity. The structure of the excretory system of the 

 larvae of monogenetic tremiatodes and its structure in a naajority of 

 Turbellaria and particularly among Rhabdocoela serve as the basis for 

 such a conclusion. However, it is difficult to say, judging by the large 

 numbers of the trunks of the excretory system of Diplozoon and of certain 

 other Monogenoidea, whether the presence of the ciliary covering is a 



379 



