CRYPTOTAXIS 33 



the marine mammals, sea snakes, the larger 

 fishes, free-swimming molluscs and Crustacea 

 (e.g., Portunidae, the swimming crabs), may be 

 described as phanerozoic ; but we may confidently 

 assert that in all cases a definite cryptotactic bias 

 of varying intensity could be demonstrated. The 

 large Cephalopod molluscs, including the squids, 

 cuttlefishes, and octopus, actually have a crypto- 

 tactic mechanism, the ink-sac, by the compression 

 of which a black fluid is discharged, effectually 

 covering the retreat of the individuals. 



Nautilus, with its great external chambered 

 shell, has no ink-sac, but lives in deep water 

 and presents a scheme of pigmentation which 

 secures its partial invisibility ; the upper part 

 of the body has the form of a fleshy hood from 

 the dark-brown surface of which arise whitish, 

 wart-like prominences giving a mottled colora- 

 tion to the exposed part of the body, which 

 harmonises well with the zebra-like markings on 

 the shell. These simulate the play of light upon 

 the surface ripples of the sea, whence I called 

 them "ripple -markings." On one occasion I 

 accidentally dropped a healthy Nautilus overboard 

 in four or five fathoms of very clear water in 

 Sandal Bay, Lifu (Loyalty Islands), and, not- 

 withstanding the translucency of the water, it 

 disappeared instantaneously from view, and baffled 

 all the efforts of an expert native diver, who was 

 with me on the raft, to recover it, the alternate 



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