252 RECORD OF CURRENT RESEARCHES RELATING TO 



it never falls to zero, so that the possibility of vision is not excluded, 

 and the eyes of animals need only to be adapted to the comparative 

 darkness. This appears to be the case with the Casj^ian Mysidae, 

 G. caspius, Boeckia, &c. It is, however, conceivable that in many 

 animals the ejes do not become developed in the persistent darkness, 

 and are replaced by other organs of sense. In the latter case the 

 eyes may even become degenerated, and the more rapidly and com- 

 pletely the less they are used, the less the service they are capable 

 of rendering to their possessor. 



Niphargus caspius, for example, has comparatively very highly 

 developed organs of smell and touch on its antennaa, and these are 

 more numerous in the male, in which the eyes are smaller than in the 

 female. These organs may enable the animal to dispense with eyes 

 in the dark depths it inhabits, and they are thus in course of degene- 

 ration, although they have not yet completely disappeared — in part 

 perhaps to be made use of in ascending to 35 fathoms. Onesimus 

 caspius has highly developed though concealed gustatory and tactile 

 cylinders developed on the maxillipedes ; but no sense-organs on the 

 antennaj and other external parts of the body as in Niphargus. 



Thus we see that in these two genera the defective faculty of sight 

 is replaced by the augmented functions of other organs, and even 

 brought about thereby in so far as these render the eyes not indispen- 

 sable and their retrograde metamorphosis therefore possible. How it 

 happens that in the different genera different organs come to greater 

 develoi)ment depends on the external conditions and mode of life of 

 the animal, which are to be looked upon as the primum movens in the 

 process of the degeneration of the one and development of the other 

 organ. Thus whilst Nipliargus swims freely in the water and not in 

 the mud, Onesimus burrows about in the muddy bottom. Antennje 

 with sensitive organs are of no use, therefore, and the more concealed 

 parts of the body had to be provided with such organs. 



Systematic Arrangement of the Platyscelida.* — Professor Claus 

 proposes the following arrangement of these aberrant Amphipods, in 

 the study of which he has been engaged for some time : — 



A. Body broad and com- 

 pressed ; abdomen narrowed 

 and folded under the thorax. 

 Femoral plates of tlie fifth 

 and sixth thoracic appen- 

 dages form broad plates. 



B. Body elongated, and 

 more or less compressed. Ab- 

 domen elongated and never 

 more than incompletely 

 folded under the thorax. 

 Femoral plates smaller and 

 more elongated. 



1. Abdomen much sliortened, and completely 

 folded. Mouth organs broad and compressed. 



TyphidcB. 



2. Abdomen not so short and less completely 

 folded under the thorax. Mouth organs elongated. 



ScelidoB. 



' 3. Body more or less compressed, abdomen 



greatly developed and half folded. Femoral plates 



of the fifth pair of legs large, of the sixth pair 



greatly broadened. Pronoidcs. 



\. Body hyperioid in form, abdomen as in 3. 



Femoral plates of fifth and sixth pair of legs 



triangular and similar. Lycceidce. 



5. Body elongated, abdomen large not folded. 



Femoral plates of fifth, sixth, and seventh pair of 



legs triangular, thin, but of some size. 



Oxycephalidde. 



Arbeit. Zool.-Zoot. Inst. Wiirzburg,' ii. (1879) p. 147. 



