674 
ZOOLOGY: W. J. CROZIER 
It is significant that the secretion comes mostly from the ventral sur- 
face of the mantle. When this nudibranch is disturbed, its first reaction 
is a general contraction, in which the mantle edge is extended and made 
prominent by internal fluid pressure ; the edge of the mantle is invariably 
the region which is bitten at by fishes, and it is only the part which is 
damaged in the small number of mutilated specimens found in the field. 
There exists here, I believe, an interesting case of adaptive correlation. 
As excreted, the repelling material contains globules of the blue skin 
pigment, which seems to be accidentally included in the discharged 
substance; but it is mainly a coagulated white substance holding oily 
globules. It is this substance which is the essential repellant. 
There is still another phase of the matter, which is the most significant 
of all. A striking characteristic of these nudibranchs is the curiously 
penetrating, disagreeable odor which attends them even when they 
are undisturbed. To this is to be assigned responsibility, I believe, 
for the facts, (1) that cloth bags containing Chromodoris are avoided 
by fishes; (2) that blinded fishes avoid them; and (3) that animals never 
normally in contact with Chromodoris, such as the dogfish, remora, 
and the hawksbill turtle, will approach this nudibranch closely, then 
quickly retreat without touching it. The olfactory locus of these re- 
actions seems probable, but it does not necessarily involve the idea of 
Earning odor.' 
In the present instance we have, in my opinion, positive proof for the 
validity of the theory of immunity coloration developed by Reighard. 
According to this view, startling colors are in many (if not in most) 
cases ''conceived to have arisen through internal forces under immunity 
of the organism from the action of selection on its characters.'' The 
immunity of Chromodoris zebra is determined by the repugnatorial char- 
acter of skin secretions under the control of its nervous system, and is 
further made effective by a repelling odor, which we have reason to 
believe stimulates the olfactory organs of fishes at least. Its success 
in maintaining itself as a plentiful inhabitant of the shoal waters of a 
'coral' reef region is conditioned by this immunity and by the unpal- 
atable nature of the jelly surrounding its egg strings; the slow rate of 
development of its eggs is counterbalanced by the fact that the animals 
reproduce throughout the year. 
The coloration of C. zebra is a metabolic accident, at least in relation 
to its protection, for a single experience with a normally colored speci- 
men is sufficient to cause snappers, turbots, and groupers to have noth- 
ing to do with subsequent individuals offered to them, even though 
these individuals are stained red or blue. 
