some Freshwater Amphijwds. 



241 



eye breaks up into a variable number of scattered elements 

 which occupy a considerable space without any definite 

 arrangement. 



From what has been said it appears that we must consider 

 the structure of the eyes in Bathyonyx as representing- the 

 first stage ot" degeneration, and therefore this genus as the 

 first which could arise from the Gaminarus-\\\A ancestor with 

 normal eyes. The ancestry of the blind species of Gammarus 

 and of Bathyonyx may be represented as follows : — 



1. Species of Gammarus with eyes. 

 Gamitiarus jmIcx, Jiuviatilis, &c. 



2. The forms with re- 

 duced eyes mentioned by 

 Schneider and Moniez 

 {G.pulex var. siibterra- 

 neus, Scb n. , G. flu viatUis 

 var. d'Emmerm, Men.). 



4. Species of Gamma- 

 rus without eyes {G.J'ra- 

 c/ilis, Chilton). 



3. Bathyonyx, Vejd. 

 {B. de Vismed, Vejd.). 



An exactly similar series can be made out in Crangonyx, 

 and adopting the same method as above the following result 

 is obtained : — 



1. Species of Crangonyx with eyes. 

 Crangonyx gracilis, recurvus, &e. 



2. C. compactus 3. C. suhterra- 4. Blind species 5. Boruta 



Chilt. (With 2 ^iews, Bate. With in >'orth America. Wrzes. (^. '("e^c- 



to 3 crystalline pigment-veil. brarum). 

 cones.) 



For J^iphargus also a corresponding series of species can 

 in all probability, be drawn up in spite of the fact that no 

 species bearing eyes are yet known. That such must have 

 existed, however, is proved by the species with rudimentary 

 visual organs. 



The series in this case may be shown in the following 

 manner : — 



1. j^iplmrgns (hypothetical, species with eyes). 



2. N, elegans. Garb. (With 

 little pigment-flecks.) 



3, Eyeless species. 



(«; With optic ganglia. 

 N. Kochiamis, Bute. 

 N. Casjjory, Pratz. 

 {h) Without uptic nerves 

 N. puteanus, &c. 



