the posterior end of the attaching disc and they fall off during the early 

 stages of postembryonic development together with this portion of the body. 



Finally, during the period of postembryonic development, cer- 

 tain species (Diplectanum) form special disc-shaped growths on the ventral 

 and dorsal sides of the anterior part of the attaching disc and these growths 

 are equipped with chitinous stick-shaped plates or chitinous rings which 

 serve as auxiliary organs of attachment. Analogously to this, apparently, 

 a secondary attaching disc is formed in Acanthocotylidae among which the 

 primary disc loses its attaching signficance soon after the emergence of 

 the larva from the egg, although it is preserved during the entire life of 

 the worms. 



During the time of the postembryonic period the development 

 of internal organs takes place with different speed among various types, 

 and what is most interesting is that very often the individual in the process 

 of development, which has not yet reached its final form and in which a 

 number of parts of the attaching apparatus are still undeveloped, already be- 

 gins to produce eggs which are fully capable of further development. This 

 is observed especially frequently annong representatives of the genus 

 Dactylogyrus. Interesting contrary data on postembryonic development of 

 Dactylogyrus vastator Nybelin were obtained by N. A. Izumova. According 

 to her materials, a fully developed copulatory apparatus appears after 3 to 

 5 days in the larvae which settle on the gills of the host ( and at the same 

 time also the attaching armature of the disc appears to be fully formed). 

 However, the sex system of D. vastator is fully formed only on approximately 

 the tenth day, and from that time the worms begin to produce eggs. Izumova 

 succeeded in showing that the temperature of the surrounding medium has 

 great influence on the development of the sex system and also on the attaching 

 armature. Thus the larvae, the development of which took place in a temper- 

 ature of 12to 15 degrees, showed gradual change into the mature state. The 

 copulatory organ and attaching armatures reached full development only in the 

 fifth and sixth day. At a temperature of 18to 22 degrees the character of 

 the development of chitinous elements differed considerably. Thus, separate 

 elements of the attaching armature were completely formed much earlier -- 

 2 to 3 days; just as the copulatory apparatus was formed at this period although, 

 just as during the development under lower temperatures, the mature stage 

 occurred not earlier than 7 days and in this fashion the process of spermato- 

 genesis and oogenesis fell behind the development of chitinous parts. 



In conclusion one must note, however, that there are almost no 

 observations in our materials nor in the literature concerning the periods 

 of postembryonic development, which understandably exceedingly complicates 

 the comparison of existing data. Separate information on this question is 

 given by us in the "Appendix" (page 138). 



107 



