some Fresh icater Amphipods 



241 



eye breaks up into a variable number of scattered element3 

 which occupy a considerable space without any definite 

 ariangeinent. 



From what has been said it appears that we must consider 

 the structure of the eyes in Batliyomjx as representing the 

 first stage of degeneration, and therefore this genus as the 

 first which could arise from the Gammarus-XWie. ancestor with 

 normal eyes. "I'lie ancestry of the blind species of Gammavas 

 and of Bathyonyx may be represented as follows: — 



1. Species of Gammarus with eyes. 

 Gammanis pulex, Jluviatilis, &c. 



2. The forms with re- 4. Species of Gamma- 



duced eyes mentioned by rus without eyes (&. fra- 



Schneider and Moniez </iiis, Chilton). 



(G. pulex var. suhfcrra- 

 neus, Schn., G. fluviatilis 

 var. d'Emmerin, Mon.). 



An exactly similar series can be made out in CmngunyXj 

 and adopting the same method as above the followino- result 

 is obtained : — 



1. Species of Crangonyx with eyes. 

 Crangonyx gracilis, recurvus, &c. 



3. Bathyonyx, Vejd. 

 {B. de Vismesi, Vejd.). 



2. C. computus 3. C. suhferra- 4. Blind species 5. Boruta 



Chilt. (With 2 mms, Bate. With in North America. 'Wnes. (B.' tenc- 



to 3 crystalline pigment-veil. braruin). 

 cones.) 



For JSiphargus 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 followiii"- 

 manner : — " 



1. ^iphargus (hypothetical, species with eyes). 



2. N. elegans, Garb. (With 

 little pigment-flecks.) 



3. Eyeless species. 



(a) With optic ganglia. 



N. K(H-/iianits, Bate. 



y. Cas/Kiry, Pmtz. 

 if)) Without opt if nerves, 



N. iititcnmif, ii-. 



