136 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



We will now turn to the following statement as to the condition of the eyes in the Crustacea 

 of the Caspian Sea 



O. Grimm enumerates several instances of Amphipods which, live in depths from 35 to 250 fathoms in the Cas- 

 pian Sea, and which, nevertheless, are provided with eyes in different degrees of perfection, some with red pigment, 

 some without pigment. Niphargus caspius has very small eyes with dark pigment and much-developed cylindrical 

 organs for smelling and touching (scarcely for hearing) on its upper antennse; Onisimus caspius has eyes without 

 pigment and much-developed cylindrical organs for touching hidden in the external plates of the maxillipeds, etc. 

 He comes to the conclusion that in depths of 100 meters and upwards there is no absolute darkness, and that in such 

 depths some animals are provided with very large eyes, others with imperfect eyes, hut in compensation with other 

 highly developed sensitive organs on various parts of their body.* 



If there is no absolute darkness at such depths it is not said that there is not more or less 

 obscurity, such as reigns in the Swiss lakes at a depth of 250 meters, and the fauna here described 

 may be compared with the twilight fauna of caves and the fauna of wells where more or less light 

 penetrates. 



We now come to the deep-lake fauna of Switzerland, which has been so fully discussed in all 

 its bearings by M. Forel and others. As regards the depth to which light penetrates, the carefully 

 conducted observations of Prof. F. A. Forel, of Geneva, made upon the Lake of Geneva in 1874, 

 proved, at least as far as the resources of photography and the human retina permitted, that the 

 limit of absolute darkness in that lake was reached in summer at the very moderate depth of 45 

 meters and in winter at 100 meters. Under normal conditions of sight a shining object disappeared 

 when immersed below 16 to 17 meters. Asper, who continued the researches of Forel upon the 

 Lake of Zurich, found in 1881 that photographic plates sensitized with bromide of silver emulsion 

 indicated the penetration of light to at least 90 meters. But while the researches here reeoided 

 fix the limit of luminous perception as dependent upon the powers of the human ntina, they do 

 not necessarily determine the same for the retina and visual nerves of the lower animals. Indeed, 

 the presence of well-developed eyes in many of the animal forms inhabiting the greatest depths, 

 no less than the varied coloring of their teguments, have frequently been taken in evidence to 

 prove not only the existence of light there, but also the unequal visual powers of the different 

 organisms. 



A writer in the Nation, New York, states as follows: 



The recently conducted investigations of a special committee of Swiss scientists, among whose names we find 

 those of Sarasin, Soret, Pictet, C. De Candolle, and Fol, seem to affirm in a general way the conclusions reached by 

 Forel; namely, that luminous penetration extends to only moderate depths. Three caudles (contained in a lantern) 

 immersed in the clearest water of the Lake of Geneva were visible at a depth of 30 meters, and an electric light at 3 

 meters farther. The distance of clear vision was found to be but very feebly dependent upon either the increase of 

 brilliancy in the luminous body or its absolute magnitude. The extreme limit of the sun's luminous action wa.s 

 determined photographically to be 250 meters, beyond which absolute darkness was supposed to prevail. 



Nature for November 20, 1884, p. 72, also contains a report of the Academy of Sciences, Paris, 

 on the depth to which sunlight penetrates the waters of Lake Geneva, by M. M. H. Fol and Ed. 

 Sarasin. From a series of experiments carried out in August and September of that year the 

 authors conclude that light reaches a depth of 170 meters, and probably a little more, the lumi- 

 nosity at this point being about equal to a clear moonless night. 



The deep-lake fauna of Switzerland as enumerated by Du Plessis, besides Protozoa, also 

 common in shoal water, and the fresh-water sponge, a flesh-colored Hydra, several Polyzoa, and 

 snails, as well as Platelminthes, includes Planaria cavatica Fries of caverns, which is white and 

 ordinarily eyeless, though a goodly number had very small eyes. Among the oligochete worms 

 occur Lumbriculus pellucidus Du Plessis, widely spread throughout numerous lakes and inhabiting 

 a depth as great as 1,009 meters. The Entomostraca belong to mostly well-known species, though 

 a purely bottom inhabitant is Moina bathycola Vernet, and perhaps a variety of M. brachiata or 

 Monoculus brachiatus Jurine. The truly blind Crustacea are Asellus forelii Blanc, probably a 

 variety of A. cavaticus of subterranean waters, and Niphargus puteanus Koch. 



* Arch. f. Nat., xlvi, pp. 116-126 ; translated, Ann. Nat. Hist. (5), v, pp. 82-92. 



