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 affectino- 

 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 fiuictional e3'es 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 Notostoini and Guatho- 

 phausife, 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 

 fi'om above, or the pale phosphoric gleams evoked by their j^assage 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 t has noted this, and the late Pro- 

 fessor J. Wood-Ma.son specified many such cases, in his account of the Cru.s- 

 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 Cliun, Die pelagiselie TliierweU, etc., 

 p. 59 (Biblioflieca Zoologica, Erster Band, Heft 1, 1888). 

 t Bull. Mus. Comp. Zool., XXIII. 82, 1892. 



