No. 628] ONCHIDIUM AND ADAPTIVE COLORATION 429 



SUMMABY 



VJ. Oucli'idium floridmum Dall exhibits at Bermuda 

 two chief types of pigmentation, a pale type tending to 

 dull olive yellow, which tends to be concealingly colored, 

 and a much more abundant type of dark blue-black ap- 

 pearance. 



No correlation can be established between the pig- 

 mentation of an Onchidium~vih\Qh. there is some reason 

 to consider the result of genetic factors primarily— and 

 the hue of the substratum over which the snail creeps in 

 the open at low tide. 



0. floridanum possesses repugnatorial mantle-glands 

 of an effective type, secreting a granular emulsion of sub- 

 stances having a strongly acid reaction and producing on 

 moist surfaces of the human mouth a pronounced sting- 

 ing sensation. Touch and pressure stimuli on the dorsal 

 surface of the mantle are characteristically involved in 

 releasing the discharge of these glands, which shoot their 

 contents, in the form of a fine spray, to a distance about 

 ten times the length of the OncMdium and with conspicu- 

 ous accuracy of direction toward the source of excitation. 



These facts are incompatible with the view that the col- 

 oration of OncMdium is determined or controlled by 

 selection in the direction of homochromicity or conceal- 

 ment. Nor can they be understood in terms of ''warn- 

 ing" coloration. 



1918. The "Homing-Habits" of the Pulmonate Mollusk OncMdium. 

 Proceed. Nat. Acad. Sci., Vol. 4, pp. 319-321. 



Heilprin, A. 



1889. The Bermuda Islands. Phila., vi + 231 pp., 17 pi. 

 Joyeux-Laflfuie, J. 



1882. Organization et d^veloppemente de POncidie. Oneidium celti- 

 cum Cur. Arch. Zool. exp^r., Tom. 10, pp. 225-383, pi. 14-22. 

 Longlej, W. H. 



1817. Studies upon the biological significance of animal coloration. 



I. The colors and color changes of West Indian reef-fishes. 

 Jour. Exp. Zool., Vol. 23, pp. 533-600, 1 pi. 



