cavity can remain sufficiently large or be considerably reduced in size. 

 These concavities, which were formed in a secondary fashion, can be con- 

 sidered as male or female atria (^atrium masculinum and atrium feminiurn). 

 Independently from the isolation of the divided atria, a chitinous armature 

 in the shape of varying forms of hooks analogous and perhaps even homol- 

 ogous to those of the male copulatory apparatus is formed on the upper in- 

 terior surface of the common sex atrium among many more highly organized p. 55 

 types, especially among Microcotylidae. These hooks, which are charac- 

 teristic for the separate species, can be located directly on the upper wall 

 of the atrium or in the special concavity or even in a special muscular sex 

 papilla separated by a special membrane from the surrounding tissues but 

 not connected with the terminal part of the male sex ducts. 



The male sex system among monogenetic trematodes is repre- 

 sented by well -developed seminal ducts, seminal reservoirs, supplementary 

 glands, and a copulatory apparatus. 



The male gonads for the most part are in the shape of rounded 

 bodies, more rarely they are lobulated or of some other form. The 

 number of testes varies but the basic number is one. The opinion of Fuhr- 

 mann that the two testes appear to be primary is completely faulty and is 

 based on an analogy with digenetic trematodes. Within limits of the sepa- 

 rate groups the number of testes is smaller among the most primitive forms 

 than among the highly organized ones. Thus among Dactylogyrus and, close 

 to it, Ancyrocephalus and others, the number of testes always equals one. 

 In the species Parancyrocephaloides daicoci Yamaguti, a form which is 

 close to the ones mentioned above, there is also one testis but it is bifur- 

 cated from the posterior end almost to the anterior edge (Fig. 79). The p. 56 

 fact that we deal here with the beginning of the bifurcation of the testis and 

 not with the reverse process is substantiated by the presence of a single 

 seminal duct emanating from the anterior end of the testis. Finally, among 

 the more highly organized form Linguadactyla molvae Brinkmann, related 

 to the same group, the number of gonads is considerable (Fig. 21). With- 

 in the limits of the aberrent group, Microbothriidae, there is one testis 

 among Leptobothrium and Leptocotyle , there are two lying symmetrically 

 side by side among Dermophthirius and finally, many in the shape of 

 follicles closely pressed to each other among Microbothrium . Among Mono- 

 cotylidae there is a number of genera having from one to a multitude of 

 testes. Among representatives of Heterocotyle and Dasybatotrema there 

 is only one testis (Fig, 80), Empruthotrema raiae MacCallum has a testis 

 bifurcated from the anterior end almost to the posterior end or, to be more 

 precise, folded in two because the seminal duct emerges from one of the 

 anterior ends (Fig. 15). Among the species of the genus Dionchus there 

 are two testes lying one behind the other (Fig. 16), and among Monocotyle -- 

 three, of which one lies behind and two symmetrically side by side and p. 57 

 closely contiguous in front of the rear one (Fig. 27). Finally among many 

 genera, as for instance Calicotyle there are nunnerous testes (Fig, 81), 



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