hesitation place such a form in the same group as Rhabdocoela, which 

 agrees with the commonly accepted view of the origin of monogenetic trema- 

 todes. The development of the chltinous armament of the posterior end of the 

 body, which assume the function of attachment at the expense of a decrease 

 in the role of the adhesive glands attaching to the substratum, which are so charac- 

 teristic of Turbellaria, can, by themselves, serve as the basic character indicating 

 the formation of tlie new group. 



We can clearly 

 divide the larvae of monogenetic 

 trematodes into two basic groups 

 differing by a number of charac- 

 ters and at the same time charac- 

 teristic for two morphologically 



..N\\\\\\ ///-■■//. 



p. 98 



Fig. 114. Hypothetical free -swimming 

 larva of promonogenetic trematodes. 

 Explanation in text. 



Fig, 115. Hypothetical promono- 

 genetic trematodes. Explanation 

 in text. 



different groups of adult Monogenoidea. To the first group are related the 

 larvae of eight families— Dactylogyridae, Diplectanidae, Calceostomatidae , 

 Tetraonchidae, Tetraonchoididae, Capsalidae, Polystomatidae, Sphyranuridae. 

 And to the second,the remaining five --Mazocraeidae, Diclidophoridae, 

 Discocotylidae, Microcotylidae, and Diclybothriidae, The last family, 

 however, has a number of distinctive traits which place it somewhat in 

 isolation (see further page 404 ). 



99 



