EYE-COLOUR IN GAMMARUS. 

 By E. J. ALLEN, D.Sc, F.R.S., and E. W. SEXTON, F.L.S. 



From the Laboratory of the Marine Biological Association, Plymouth. 



(With Plate XIV and One Diagram.) 



CONTENTS. 



PAOE 



I. Colour associated with the Perfect Form 347 



II. Absence of Colour associated with Imperfect Form . . . 349 



III. The No-white Mutation 350 



IV. Spots 352 



(1) The Appearance of Spots in the Pure Bed Stock . . 354 



(2) Appearance of Spots in the Albino Stock . . . 355 



(3) Selected Cases 356 



(4) Attempts to establish Pure Stocks .... 358 



A. Experiments to establish an Unspotted Strain . . . 358 



B. Experiments to establish a Pure Spotted Strain . . . 361 



1. Albinos mated with Keds 361 



2. Tested Albinos mated with tested Albinos . . . 363 

 v. Spots with the No-white Mutation ...... 364 



The eyes of Gammarus chevreuad with which we are dealing are of 

 two kinds, derived from the same stock, a Perfect Form with Colour, — 

 the " Normal " eye — and an Imperfect Form, with no colour, which we 

 call the " Albino '" eye. (See Allen and Sexton, Joum. Marine Biol. 

 Assoc, Vol. XI, No. 3, Dec, 1917, p. 274.) 



I. Colour associated with the Perfect Form. 



In the wild state the eye is reniform in shape, with the margin 

 entire, and consists of a large number of ommatidia arranged in regular 

 rows, the number increasing with each moult from 10 to 12 in the 

 newly hatched to 70 or 80 in the adult. (Plate XIV, Fig. 1.) 



Each ommatidium is surrounded by five deeply pigmented retinal 

 cells. The retinal pigment in the wild animal is black, but a mutation 

 occurred in the second generation from adult animals brought into the 

 laboratory. In this mutation the eye-colour was red. In addition to 

 this pigment, there is a superficial layer of chalky-white pigment spread 



