78 



host for their own further development, and no doubt, a considerable number of the larvae 

 of all species are destroyed while swimming about, partly because many of them cannot find 

 their object in due time. Then again, there must be circumstances which cause a compara- 

 tively smaller percentage of the brood of the least prolific species, and an enormous percen- 

 tage of the brood of the most prolific species, to be destroyed during this period. This, 

 again, must be supposed — at least partly — to have something to do with the difference 

 of the number of specimens of the species which constitute the hosts. Now, as the larvae 

 of the most prolific species seek large forms, those of the least prolific small forms, and as 

 the large forms, as we know, are found on an average in much smaller number than the 

 small forms, it follows that the larvae by which they are sought, have as a rule much less 

 chance of finding them in due time, for, as previously stated in detail, there is no considerable 

 difference in the structure and size of the different larvae, — e. g. the larva of Choniostoma 

 mirabile is only about one eighth longer than that of Stenothocheres egregius, but, as far as I can 

 see, scarcely so vigorous and so well adapted for swimming, (comp. the peduncles of the natatory 

 legs in the two species; pi. I, fig. 1 1, and pi. XI, fig. 1 e). — On the base of my material 

 of parasites and of my knowledge of the biology of the hosts, I might set forth several points, 

 thus giving a wider scope to the discussion of these matters, but for various reasons I 

 abstain from doing so. 



C. About Classification. 



a. Limitation and Characters of the Species. 



Of small Crustacea, such as Cladocera, Ostracoda and free-living Oopepoda, there 

 are in most cases some or many specimens of each species at the student's disposal for 

 determination of the forms, and even where these animals are so small that the compound 

 microscope has to be used in order to determine them, most of them can stand the pressure 

 of a glass- cover, and as a rule it is unnecessary to submit the specimens to much parti- 

 cular preparation, except where a description of them has to be given; finally, most species 

 have a very fixed shape. All these factors help to facilitate the determination of the species. 

 In Choniostomatidae the circumstances are different. The animals are so rare, that of most 

 species only a single specimen or a few specimens of each sex can be procured; neither 

 males nor females can bear the pressure of a glass-cover; the males are so small, that they 

 cannot be examined without high magnifying power, and before the examination particular 

 care has often to be taken in placing them in the preparation, and though the females are 

 much larger, the parts of their body which have to be investigated are exceedingly small 



