Speaking of the buccal opening and its relation to the cephalic 

 glands, one must not forget that when we indicate two groups of cephalic 

 glands we mean the glands themselves and not the clusters of their 

 effering ducts, the number of which could be greater but is usually 

 paired. There are, however, a few exceptions when in addition to the 

 paired groups of glands, there is one unpaired, lying terminally and 

 opening by one cluster of ducts at the anterior end of the body. 



It is not possible to characterize the sex apparatus for the 

 subclass as a whole--so varied in its structure, especially since 

 viviparous forms also enter into the given subclass, 



The representatives of the subclass parasitize fishes related 

 to Elasmobranchii, Holocephali (as an exception), and Teleostomi (basic 

 mass). In addition to that they are widely distributed among the 

 Amphibia, Reptilia and Mammalia (?) and they are also encountered on 

 cephalopods. Thus, if one does not take into consideration the para- 

 sitizing of the crustaceans .which clearly is of secondary nature, the 

 subclass is scattered in all groups of hosts from which monogenetic 

 trematodes are known at the present tinme. 



This subclass includes all families which pertain to Mono- 

 pistocotylea according to the systems of Price and Sproston, and part 

 of which also are related to Polyopisthocotylea, i. e. , it principally p. 344 



differs from the classification of Odhner because it amalgamates forms 

 which have and do not have a ductus genito-intestinalis. 



The grouping of the forms relative to this subclass is based 

 first of all on the changes of the attaching apparatus which are connected 

 with the changes in the nature of adaptations towards parasitizing on the 

 body of the host. The basic trends of changes can lead in the following 

 three directions: 1) the preservation and development of the chitinous 

 armature in all stages of development; 2) the preservation of the 

 chitinous armature used during the first stages of development, and its 

 functional replacement later by a disc which changes into a powerful 

 sucker; 3) the preservation of the chitinous armature functioning during 

 the first stages of development with its subsequent replacement by a 

 number of suckers formed anew on the attaching disc (see page 328 ). 



With this, one must remember that during the systematization 

 of the forms which pertain to this group on a phylogenetic basis one must 

 bear in mind that the evolutionarily separate forms and their groups can 

 "arrest themselves" at different stages of adaptation within the limits 

 of the basic trends of development and>in such a fashion convergent 

 similarities are possible in any of the three trends or directions; 



406 



