78 



host for their own fiirthei' develoimieiit. mid no doubt, a considerable niiniher of the larvae 

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

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

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

 tage of the brood of the most jjrolitic si)ecies, to be destroyed dnring this jieiiod. This, 

 again, must he supposed — at least partly — to have something to do with the dilierence 

 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 Ave know, are found rut (in 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 difterent larvae, — e. g. the larva of Choninstoma 

 mirabile is only about one eighth hmger than that of Sfenothocheres effregiun, but, as fai- 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 matteis, but for various reasons I 

 abstain from doiug so. 



C. About Classificatiou. 



a. Limitation and Characters of the Species. 



Of small Crustacea, such as Cladocera, Ostracoda and free-living Copepoda, 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 diflerent. The animals are so rare, that of most 

 species only a single specimen oi' a few specimens of each sex can be produced; 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 



