250 M. A. Humbert on Niphargus puteanus, var. Forelii. 



this organ has, in the first lialf of its length, a chitinous wall 

 of a certain thickness ; and the author says that it is larger in 

 the adults than in the young, and chitinized throughout its 

 length, and then more resembles the organs of this nature 

 that are met with in insects. 



I do not know of other figures or descriptions that may be 

 referred to these organs. It is very possible that they have 

 been described in a memoir by Jarschinski*, which I have 

 been unable to consult, and of which I must content myself 

 with reproducing the title from the ' Zoological Record.' 



On the inferior antennee sensitive set^, hyaline bacilli, and 

 auditory setce are also met with. 



The buccal organs and the feet present extremely varied 

 forms of set£e, the arrangement and number of which exhibit 

 a remarkable constancy in the two varieties which have been 

 comparatively studied. 



This is not the place in which to enter into further details 

 as to the structure of these Crustacea. I shall content myself, 

 in conclusion^ with reproducing the paragraphs in which I 

 have treated of the habitat of the N-ijjhor^us of the Lake of 

 Geneva, and discussed the problem of tlie origin of those 

 crustaceans which inhabit waters destitute of light. 



" What is the origin of the blind Gammarids which we 

 find in wells, in caverns, and in the depths of seas and lakes ? 

 Such is the problem which cannot fail to force itself upon 

 the minds of all who investigate these crustaceans. Two dif- 

 ferent solutions of it may be given. It may be assumed that 

 these animals were created such as they are now, because, 

 being destined to live in places deprived of light, they had no 

 occasion for visual organs. This explanation, or, rather, this 

 reply, formerly the only one admitted, now-a-days satisfies only 

 a very small number of naturalists ; and many powerful argu- 

 ments may be brought to bear against it. It will be suffi- 

 cient to cite a single one, namely the fact that otlier animals 

 living under the same conditions of obscurity are furnished 

 with perfectly organized eyes. Thus certain Gammari of 

 Lake Baikal, living between 50 and 500 metres, have the 

 eyes well constructed and furnished with black pigment. 

 The Munidce^ which are dredged in the sea at depths of 

 1000-1200 metres and more, have the eyes exceptionally 

 developed and apparently extremely sensitive. The Onatho- 

 jphausice dredged by the naturalists of the ' Challenger ' 

 between 1830 and 4020 metres have normal pedunculated 



* F. Jarschinski, *'0n the Leydigian organs of the antennae of the 

 Crustacea Amphipocla," Premier congres des natiu'alistes Russes a St. 

 P^tersbourg, 1868, pp. 311-318 (in Russian). 



