as the complication "of the suckers" is observed. Thus, among many 

 Capsalidae the relative number of the glands sharply increases 

 (Nitzschia and others) and among the very independent forma of Loimos 

 the head pair of suckers forms a greater number of them secondarily 

 (see page 20 ). 



Egg laying species. The sex systems of the species per- 

 taining here are very diversified, just as is the structure of the sex 

 armature. Two tendencies of the development of the latter are charac- 

 teristic; in the direction of the strengthening of the chitinous apparatus 

 and sometimes in its considerable complication and- -in the direction of a 

 gradual reduction of the chitinous parts and their replacement by purely 

 muscular formations. Within the limits of the order and a number of its 

 families one can observe the increase in the number of testes starting 

 from the initial single one up to a very large number. One cannot fail 

 to note with this that the number of seminal effering ducts does not 

 correspond to the number of testes and therefore, it is not quite correct 

 to regard this process as polymerization (Dogiel, 1954a). 



The representatives of the order parasitize only fishes, 

 mainly Teleostonoi and to a snnaller degree Elasmobranchii. Only one 

 species is encountered on Holocephali. 



This order includes families pertaining to the suborder 

 Monopisthocotylea according to Odhner, with the exception of the family 

 Gyrodactylidae and in this fashion corresponds to it to a certain degree; 

 however, the principles of classification accepted by us do not in any way 

 equal those established by Odhner because the latter separated the group 

 of Monopisthocotylea by the characters of the absence of the ductus genito - 

 intestinalus and the presence of a "true" vagina^- whereas^ we do not attribute 

 great phylogenetic significance to these characteristics. 



Within the limits of the order the main directions of its 

 differentiation on the basis of the structure of the attaching armature 

 and its adaptations to different ways of parasitizing the host are easily 

 noticed. One cannot fail to note that along with the progressive develop- 

 ment of certain attaching formations we observe frequent cases of reverse 

 nature, i.e. , reduction of the separate parts and organs of attachment 

 within the limits of the order. It is indispensable to keep this circumstance 

 in view all the time because we often see similar morphological pictures 

 as a result of convergence during different trends of evolutionary process. 

 These convergencies should never be confused with the converging 

 similarities which appear as a result of the parallel development of 

 separate branches of the phylogenetic tree. As will be seen from that 

 which follows, convergencies based on parallelism during development 

 have a wide distribution in the group under study. 



408 



