INHERITANCE OF EYE-COLOUR IN GAMMARUS. 21 
A word of explanation is necessary as to the terms employed in this 
paper. The Black eye-colour is dominant to the Red, and therefore Red 
is referred to as Recessive (R.). Black divides into Pure (P.) and Impure, 
but instead of the term “impure ” the word Hybrid (H.) is used. 
THE PIGMENTATION OF THE EYE. 
The structure of the eye of Gammarus has been well described and 
figured by Parker (“The Compound Eyes in Crustaceans,” Bull. Mus. 
Comp. Zool., Harvard, Vol. XXI, Plate I), the species investigated by him 
being the Gammarus ornatus of Milne-Edwards (=Gammarus locusta, 
Linn., Stebbing, Das Tierreich, V. 21, p. 476). 
Sections of the eye of Gammarus chevreuxt show precisely the same 
internal structure. The eye in this species is reniform in the adult, oval 
in the young animal, much raised and rounded. The superficial aspect 
presents a reticulation of opaque white pigment, with the ommatidia 
appearing as coloured spots, black or red, in the spaces of the 
network. 
The black pigment of the retinular cells of the ommatidia of the Black 
eye appears to be produced by a combination of black and red, even in 
the so-called “Pure Black” animals (Fig. 1), with a larger admixture of 
the red in the “ Hybrid Blacks ” (Figs. 2 and 4). 
The pigment of the Red eye is pure red, with no alloy of the black 
(Figs. 3 and 5). 
Sometimes the retinular cells are unpigmented and the white reticula- 
tion shows up in a very striking way, giving the effect of chalk-white 
eyes—the “ All-white” eye referred to in the paper (Figs. 8, 9, 
and 10). 
Occasionally again, some of the ommatidia are pigmented and some 
not ; this variation is called the “ Part-white” eye (Fig. 7). 
The white opaque pigment is subject to great variation, sometimes 
showing as faint thread-like lines, sometimes broken up and irregular, 
sometimes present in excess, obscuring the ommatidia, and sometimes 
it is entirely lacking, the ‘‘ No-white”’ eye (Fig. 6). Animals are often 
found with one or both eyes affected. The defect can be transmitted by 
normal-eyed animals to both black and red eyed offspring. 
