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Part TIL — Seventeenth Annual Report 



some instances a considerable diversity of form, which it may be of interest 

 to refer to somewhat in detail ; and, in order to avoid as far as possible 

 confusing one thing with another, I will consider separately the Daphnice 

 observed in each of the lochs, with remarks, where necessary, on the 

 relationship of the various forms observed. 



The study of these organisms has been more difficult than that of some 

 of the other groups owing to the diversity of opinion that exists as to 

 what forms should be considered " species " and what "varieties." In 

 some instances the tendency to variation is so great that though extreme 

 forms when compared directly with each other seem to be "good " species, 

 yet gradations between them may be met with, which prove more or less 

 clearly that the two extreme forms are merely " forms " of one species ; this 

 tendency to variation, which is specially evident in Daphnice from some 

 of the lochs recently examined, makes their study both interesting and 

 perplexing. In my notes of the Dajrtinice of the different lochs, I will be 

 guided to some extent by Professor G. S. Brady's " Revision of the 

 British Species of Daphnia and other Allied Genera," and the same order 

 will be observed here in dealing with the lochs as is observed in the 

 former part of this paper. 



It may be remarked further that the Daphnice with crested heads (so com- 

 mon in some of the lochs of the middle and north of Scotland, and which have 

 been recognised as identical with Professor G. O. Sars' Daphnia galeata) 

 have, in several of my previous papers on the invertebrate fauna of the 

 inland waters of Scotland, been ascribed to Daphnia jardinii of Baird, 

 because it seemed to me that Dr. Baird's short definition of his species 

 (published in the Edinburgh New Philosophical Journal, 1857) was quite 

 applicable to some of the forms of this variable species. Dr. Brady and 

 also Professor G. 0. Sars appear, however, to regard Daphnia galeata 

 and Daphnia jardinii as distinct ; yet, as Daphnia galeata shows so 

 great a tendency to variation, it will not be very surprising if the form 

 to which Dr. Baird's name has been restricted should, after all, prove to 

 be but another variety of Sars' species, the product of some peculiarity in 

 the environment, i.e., a slight difference in the chemical properties of the 

 water, or in the general temperature, or in the food supply. But questions 

 such as these can only be satisfactorily solved by a careful and systematic 

 study of the life-history of the organisms concerned, and that can scarcely 

 be done except in a properly-equipped laboratory. 



It might be useful if names were given to even a greater number of 

 the remarkable forms of such a protean species as Daphnia galeata than 

 has yet been done. A number of the prominent varieties of this form 

 will be referred to when we come to discuss the Daphnice of Loch Ness, 

 Loch Oich, and Loch Lochy. Meantime, I go on to consider briefly one 

 or two points concerning the Daphnice observed in Loch Lomond. The 

 Daphnice of the other lochs will be referred to in their order. 



The Loch Lomond Daphnice. (Plate VII., figures 34, 35, 51.) 



All the Daphnice contained in the various gatherings from this loch 

 appear to belong to the one species — Daphnia Jacustris, G. 0. Sars. 

 Professor G. S. Brady, who kindly examined some of the specimens from 

 this loch, considered them to belong to that species. A few of the Loch 

 Lomond specimens present what appears to be an interesting departure 

 from the normal form of D. lacustris. The adult female, according to 

 the description of the species, has no tooth on the top of the head or even 

 on the dorsal aspect near the top. " The presence of a vertex tooth is a 



