primary or secondary phenomenon. It is possible that the original 

 monogenetic trematodes possessed not only clearly expressed terminal 

 nephridial cells but also undifferentiated ciliary covering in the region 

 of the excretory vessels. 



The nervous systenn of promonogenetic trematodes was 

 constructed along the type peculiar to the majority of Rhabdocoela and 

 which was preserved also among the lowest Monogenoidea. Charac- 

 teristic for it is the presence of a more or less well-developed cephalic 

 brain with three pairs of nerve trunks departing posteriorly from it of 

 which it is possible that the most highly developed were the ventral ones. 

 It is most probable that already during the early stages of the separation 

 of the group, the ventral nerve trunks formed small gangliar thickenings 

 in the posterior part of the body which served to control the attaching 

 armature. The organs of feeling were represented by dermal sensory 

 ternninals and undoubtedly by eyes. The litter apparently existed in a 

 small number from the very beginning. Most probably there were two 

 pairs of them, just as is observed among the majority of the lowest 

 Monogenoidea. It is interesting to note that the majority of contemporary 

 Rhabdocoela have one pair of eyes, which points to a greater primitive- 

 ness of i;he primary monogenetic trematodes in this respect. 



Finally, the sex system of the latter most probably was 

 similar to the one of the contemporary Dactylogyridae. There is no 

 doubt that at first the promonogenetic trematodes had only one testis 

 and not two as is supposed by Fuhrmann (see page 55). All the lowest 

 monogenetic trematodes, without exception, have a single testis and its 

 division into a greater number of separate parts is beyond any doubt ^^ 

 secondary phenomenon which is observed independently in a number of 

 groups of monogenetic trematodes. This is a very important difference 

 between monogenetic trematodes and Rhabdocoela , which usually have 



_ — 



and also of digenetic trenaatodes ! 



two testes and only in rare cases one. Just as for Turbellaria, the 

 presence of separated envelopes of the testes and of individual walls 

 in the seminal ducts is characteristic for the primary forms of mono- 

 genetic trematodes. One must supnose '"hat the primary forms already 



possessed the chitinous armature of the male sex system. The female 



1 . . 



sex system is represented by one compact ovary, ^ and vitellaria and p_ 326 



This, however, can be untrue--see page 475. 



380 



