its direction, often stopping and bumping against different objects as if 

 feeling thena with its anterior end and often attempts to attach itself by its 

 posterior end. When it finds a host the larva passively or actively penetrates 

 to its place of habitation, attaches itself and, casting away its ciliary cover- 

 ings, begins its parasitic life. However, when it fails to find its host the 

 larva does not perish at once but still swinns for a long time although it 

 already does not move its attaching disc and its armature, having lost the 

 ability to attach. 



In such a fashion, we can distinguish two periods in the normal 

 life of the larvae with different physiological characteristics which can be 

 distinguished morphologically at the same time by the condition of the attach- 

 ing disc. During the first period, the latter carries its chitinous armature 

 inside the body and only after it "cuts itself" outside and the sharp ends of 

 the hooks penetrate or protrude and begin to move actively does the larva 

 acquire the ability to attach itself to the body of the host. It is precisely 

 the moment of the "emergence" of the hooks that determines the change of 

 the larva from the first period to the second period. The time of the pre- 

 sence of the larvae in the first period varies. For the majority of Dactylo- 

 gyrus it is equivalent to two to three minutes or even shorter, whereas for 

 Nitzschia sturionis (Abildgaard)--not less than five minutes and in most 

 cases longer (from 10 to 12 minutes). Then the second, most important 

 period is considerably longer. Thus, for Dactylogyrus it is not less than 

 4 to 5 hours and for Nitzschia about 24 hours. 



The biological significance of the two periods of life of the larva 

 is very great. Actually the ability of the larva to swim actively is an adap- 

 tation for the dissemination of the species to different individuals of its host. 

 If the larva had the ability to attach itself immediately to the body of its host 

 the infestation of the individual of the parent host on which the egg was 

 developed would have increased to a considerably greater degree than the 

 infection of other individuals, which undoubtedly would not have been advan- 

 tageous fronn the point of view of dissemination and consequently from the 

 point of view of the flourishing and preservation of the species. The ex- p. 94 



istence of the first period of life of the larvae appears to be a supplementary 

 special adaptation which prevents increased infection of the same host indi- 

 vidual. Indeed, both the increased activity of the larva and its positive photo- 

 tropismplus its inability to attach- -all these do not allow it to remain on the 

 same host individual but force it to seek another. 



The above-mentioned adaptations undoubtedly play a greater 

 role among worms, the eggs of which remain on the host, than among those 

 which deposit eggs on the bottom. 



Postembryonic developnnent of the egg -laying monogenetic tre- 

 matodes has been studied somewhat better than the embryonic, but is also 

 insufficiently known. Basically, at our disposal there are data about the 

 structure of the newly emerged larvae and only to a small degree do we 



94 



