COLORS OF THE DEEP-SEA CRUSTACEA. 253 
ness in which these creatures dwell, either through chemical action or more . 
probably through a physiological process originating in the eye and affecting 
the pigment cells by a reflex action. In either case the prime cause is a 
purely physical one,— the more or less complete absence of light in the 
depths of the sea. This color, then, is to be regarded as entirely useless to 
its possessor. 
The retention of pigment and functional eyes through a long succession 
of generations among the deep-sea Crustacea implies the existence of a cer- 
tain amount of light at depths far beyond the point where it can be demon- 
* 
strated by experiments with photographic plates. Although many of 
these animals are blind and show a strong tendency to become colorless, yet 
many are endowed with very highly developed eyes and display pronounced 
colors, forming a strong contrast in both respects to the fauna of subter- 
ranean caverns. The free-swimming Crustacea from great depths are, as we 
have seen, commonly of a very bright red color and endowed with visual 
organs of a high order, while the bottom species, even those from much shal- 
lower depths, are most often pale of hue and frequently blind. This difference 
is to be explained through the different mode of life followed by the two 
classes. Powerful and active swimmers, such as the Notostomi and Gnatho- 
phausiz, are fitted by their mode of life, to take advantage of what little 
light exists at those depths, whether it be some feeble rays which penetrate 
from above, or the pale phosphoric gleams evoked by their passage through 
the deep. The more sedentary, bottom species, on the other hand, would be 
prone to conceal themselves in the mud or whatever retreat the sea-floor 
might afford. Hence the aptness of these forms to lose both their pigments 
and their sense of sight. 
Colors of the cyanic series, — blues and greens, — although not uncom- 
mon among surface and shore Crustacea, are almost never seen in the species 
from the deeper waters. It is remarkable, however, that the eggs of many 
of the red deep-water Crustacea show during the period of incubation bright 
blue or sometimes green tints. Mr. Agassiz + has noted this, and the late Pro- 
fessor J. Wood-Mason specified many such cases, in his account of the Crus- 
tacea dredged by the steamer “ Investigator.” It seems likely, therefore, 
that the blue pigments are not really lacking in the adult, but merely over- 
* Cf. Fol and Sarasin, Comptes Rendus, XCIX. 793, C. 100, and Chun, Die pelagische Thierwelt, ete., 
p- 59 (Bibliotheca Zoologica, Erster Band, Heft 1, 1888). 
+ Bull. Mus. Comp. Zodl., XXIIT. 82, 1892. 
