LAKE FAUNAS. 137 



Cj' clops (sp. undet.), Daphnia hyalina, D. mucronata, Bosmia lon- 

 gispiua, Sida crystallina, Bythotrephes longimanus, Leptodora hya- 

 lina. Of the total number of twenty-four species which were ob- 

 tained by Pavesi *" from the Italian lakes, belonging almost exclu- 

 sively to such genera as occur in Lake Geneva, only four (Daphnia 

 hyalina, D. galeata, Bosmia longispina, Leptodora hyalina) are 

 known to inhabit the Lago Maggiore, and an equal number the 

 Lago di Como. The Lago d'Iseo, on the other hand, has ten spe- 

 cies, and Orta and Mergozzo eleven each. As far as the Crustacea 

 are concerned, the Swiss lakes appear to be less rich in point of 

 species than the Scandinavian ; but they alone, with the adjoining 

 lakes of Annecy and Bourget, in Savoy, have thus far yielded any 

 variety of forms of lower organisation than the articulates. M. 

 Imhof's investigations have brought to light, as constituents of the 

 Swiss pelagic faunas, two species each of flagellate (Dinobryon) 

 and cilioflagellate (Peridinium, Ceratium) infusorians, two species 

 of true infusorians (Ejiistylis lacustris, Acineta elegans), which live 

 attached on the crustaceans, and the six rotifers already referred to, 

 belonging to the genera Conochilus, Asplianema, Anurasa, Triarthra, 

 and Polyarthra. Doubtless some of these forms will also be found 

 in the more northern and southern lakes.* 



The general characters common to the animals of the pelagic 

 region, which are the outcome of their particular mode of life, are 

 thus briefly summarised by Forel: " They must sw'im incessantly, 

 without ever being able to rest upon a solid body, and, instead of 

 any organ of adhesion, they possess a highly developed natatory 

 apparatus ; their specific gravity, which is nearly the same as that 

 of the water, enables them to swim about in the water without any 

 great muscular exertion. They are rather sluggish animals, and 

 escape the enemies that pursue them rather by their transparency 

 than by their activity ; they are, indeed (and this is their character- 

 istic peculiarity), perfectly transpai-ent, like crystals ; and only their 

 strongly pigmented black, brown, or red eye appears distinctly. 

 This nearly perfect transparency of the pelagic animals may be re- 

 garded as a mimicry acquired by natural selection ; only the animals 



* " Ann. and Mag. Nat. Hist.," December, 1883 ; January, 1884. Since 

 the above was written Imhof has identified several of the Swiss crustaceans, 

 rotifers, &c., in the lakes of Alsace-Lorraine. " Zoologisher Anzeiger," De- 

 cember, 1885. s»» 



