Amphipoda of the Caspian Se.a. 89 



flagella, more resemble the species of Gammarus, and thus 

 represent the more conservative element, which, indeed, is the 

 case with the female sex generally. On the first four joint3 

 of the five-jointed main flagellum of the upper antennas of the 

 male are very large cylindrical organs, described by Ley dig 

 and others as olfactory organs. At their free extremities these 

 cylinders present each an aperture, from which perhaps, as 

 Leydig states, thin hairs may actually be exserted ; and from 

 within a nervous branchlet penetrates into each cylinder, and 

 forms a cellular inflation (in the cylinder itself) only to 

 disappear immediately afterwards, as I have observed still 

 better in living examples of another species, namely Gam- 

 marus priscus, at Ki'asnovodsk. On the secondary flagellum 

 of Nvphargus caspius, as also on the last joint of the peduncle 

 of the inferior antennas, we find peculiar organs, constructed 

 like the olfactory pencils of N. puteamts, as described by 

 Alois Humbert : these are large and resistant rods, the some- 

 what acute extremities of which are beset with a great number 

 of very thin and long chitinous hairs. In the interior of each 

 such rod runs a nerve, which, before entering into the rod, 

 swells into a nerve-cell with a nucleus. But whether this 

 nervous branchlet breaks up into still finer ones, which pene- 

 trate into the chitinous hairs, I have been unable to see, 

 although I have employed a magnifying-power of 1500 dia- 

 meters and various reagents. From their organization I 

 should not interpret these pencils as essentially and exclu- 

 sively auditory organs, but as extremely sensitive organs of 

 touch, capable of perceiving the very slightest movement of 

 the surrounding medium. 



These olfactory and tactile (or auditory) organs, which are 

 certainly comparatively very highly developed, may enable 

 the animal to dispense with eyes in the dark sea-depths inha- 

 bited by it ; and they are thus in course of degeneration, 

 although they have not yet completely disappeared — in part, 

 perhaps, because they may still be made use of, for example, 

 in ascending to depths of 35 fathoms. 



Matters are very different with the species of Onesimus, of 

 which we may take for consideration Onesimus caspius as the 

 most typical. 



The eyes of Onesimus caspius are small, irregularly oval, 

 widely separated from each other, and completely unpigmented, 

 so that they are not at once distinguishable even under the 

 microscope. It is well known that the unpigmented eyes of 

 many Gammaridas living at great depths become reddened 

 under the action of sunlight ; but this does not occur in 0. 

 caspius. We are justified in assuming that even if the species 



