Amphipoda of the Caspian Sea. 91 



these cylinders is so distinctly marked, that we are certainly 

 justified in regarding them as tactile organs, and, from their 

 position, also as organs of taste. 



Thus we see that in the species of Xiphargus and Onesimus t 

 which are either blind or furnished with imperfectly functioning 

 eyes, the defective faculty of sight is replaced by the augmented 

 function of other organs, and even brought about thereby, in 

 so far as these render the eyes not indispensable and their 

 retrograde metamorphosis therefore possible. The question 

 now arises how it happens that in the different genera different 

 organs come to greater development ; and this question is 

 answered by observation of their life-phenomena. During 

 my dredging investigations I have observed that the species 

 furnished with sensitive antennae, such as Xipihargus caspius f 

 although living at great depths, live in the water and not in 

 the mud, which is proved not only by experiment after the 

 animals have been brought up *, but also by the fact that 

 all individuals of the Xiphargus are greatly infested by 

 Vorticellce. 



The species of Onesimus behave quite differently. They 

 live constantly in the mud of the sea-bottom, and here, bur- 

 rowing quickly like moles, seek their nourishment by con- 

 suming the mud which contains particles of organic matter. 

 As a matter of course, antennas furnished with sensitive 

 organs can be of no service to them, since not only such 

 delicate and fragile structures as the olfactory cylinders 

 and pencils, but even the coarser bristles have disappeared 

 from the outer surfaces exposed to friction against the 

 mud, as we have already stated by indicating that in Onesi- 

 mus such bristles exist only on the inner surfaces of the 

 antennas, which protect each other. But as external sense- 

 organs could not be developed, the more concealed parts of 

 the body had to be provided with such organs. We have 

 already seen that in the species of Onesimus the setae of the 

 outer lamella? of the maxillipedes are developed into sensitive 

 organs ; and although it is not yet decided whether they repre- 

 sent taste-organs, we cannot avoid regarding them as organs 

 adapted to the determination of the quality of the food, which, 

 in the subterranean life of these animals, replace the eyes, and 

 thus also bring about their retrograde metamorphosis. 



We may briefly summarize all that has been said as fol- 



* The animals brought up by the dredge were always placed, first of 

 all, in small basins of water for the purpose of the observations above 

 indicated : but the relation of the animals to the mud is to be seen even in 

 the dredge itself ; the water-animals (as opposed to the mud-animals) do 

 not bury themselves deeply in the mud, and are speedily suffocated in it. 



