HEREDITY IN PLANTS, ANIMALS, AND MAN. 365 
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Gammarus chevreuxi is a small shrimp-like animal, belonging to the 
Crustacean order, Amphipoda. It is about } inch long, and lives in great 
abundance in the brackish-water ditches at Chelson Meadow, just above 
Laira Bridge. It has never been found anywhere else. A drawing of the 
animal is shown on Plate VII, Fig. 1, which accompanies the preceding 
paper by Allen and Sexton in this number of the Journal. 
The animals are kept alive easily in glass finger-bowls and feed freely 
on dead leaves, especially on elm leaves. The eggs are carried by the 
female in a brood pouch until they are hatched. As soon as one batch of 
eggs is hatched and the young liberated from the pouch, another batch 
is laid. A batch may contain as many as 50 eggs, so that a large number 
of young can be obtained altogether from one pair of animals. The eggs 
take 14 days to hatch at a temperature of about 60° F. The young grow 
rapidly and reach maturity in about 36 days at summer temperatures. 
Hence from 5 to 6 generations can be obtained in the course of a year, 
a fact which makes the animal specially suitable for the study of the 
problems of heredity. 
The eye of Gammarus, like that of all crustaceans and insects, is of the 
compound type. It is made up of a considerable number of single 
elements, the ommatidia, each provided with a simple lens and receiving 
a nerve-fibre from the optic nerve. 
In the normal animal each ommatidium is surrounded by 5 pigment 
cells, which lie deeply in the tissue of the eye, and are filled with pigment 
of a jet-black colour. Just below the surface of the cuticle or skin and 
surrounding the black pigment there is a quantity of milk-white or rather 
chalk-white pigment. This gives the whole eye, when looked at directly 
in the living animal, a honeycombed appearance, the white pigment 
forming a kind of network in which the round, black ommatidia are en- 
meshed. (See Plate VII, Fig. 2, of preceding paper.) 
Whilst the habits and development of this animal were being studied, 
there appeared amongst the descendants of a pair of normal black-eyed 
Gammarus brought in from Chelson Meadow in June, 1912, in the third 
generation, that is amongst the grandchildren, a small number of young 
ones which had bright red eyes. The usual black pigment was replaced 
by red pigment, the network of chalk-white remaining as in the ordinary 
eyes. (Plate VII, Fig. 3, of preceding paper.) From this family a race of 
red-eyed animals was established, which has been used in these experi- 
ments. It is only in this one family that red eyes have ever appeared, 
and although very many thousands of specimens from the natural habitat 
have been examined, and many thousands more have been bred from 
pure black-eyed parents, no other case of the sudden appearance of a 
red eye has been met with. 
