298 THE DESCENT OF MAN. 



sitic worm under the name of Hectocotyle. But this mar- 

 velous structure may be classed as a primary rather than as 

 a secondary sexual character. 



Although with the Molluska sexual selection does not 

 seem to have come into play ; yet many univalve and 

 bivalve shells, such as volutes, cones, scallops, etc., are 

 beautifully colored and shaped. The colors do not appear 

 in most cases to be of any use as a protection; they are 

 probably the direct result, as in the lowest classes, of the 

 nature of the tissues; the patterns and the sculpture of the 

 shell depending on its maimer of growth. The amount of 

 light seems to be influential to a certain extent ; for 

 although, as repeatedly stated by Mr. Gwyn Jeffreys, the 

 shells of some species living at a profound depth are brightly 

 colored, yet we generally see the lower surfaces, as well as 

 the parts covered by the mantle, less highly colored than 

 the upper and exposed surfaces.* In some cases, as with 

 shells living among corals or brightly tinted sea-weeds, the 

 briglit colors may serve as a protection, f But that many 

 of the nudi-branch molluska, or sea-slugs, are as beauti- 

 fully colored as any shells, may be seen in Messrs. Alder 

 and Hancock's magnificent work; and from information 

 kindly given me by Mr. Hancock, it seems extremely 

 doubtful whether these colors usually serve as a protection. 

 With some species this may be the case, as with one kind 

 which lives on the green leaves of algae, and is itself bright- 

 green. But many brightly-colored, white, or otherwise 

 conspicuous species, do not seek concealment; while again 

 some equally conspicuous species, as well as other dull- 

 colored kinds, live under stones and in dark recesses. So 

 that with these nudibranch mollusks, color apparently does 

 not stand in any close relation to the nature of the places 

 which they inhabit. 



These naked sea-slugs are hermaphrodites, yet they pair 

 together, as do land-snails, many of which have extremely 

 pretty shells. It is conceivable that two hermaphrodites, 



*I have given ("Geolog. Observations on Volcanic Islands," 1844, 

 p. 53) a curious instance of the influence of light on the colors of a 

 frondescent incrustation, deposited by the surf on the coast-rocks of 

 Ascension, and formed by the solution of triturated sea-shells. 



f Dr. Morse has lately discussed this subject in his paper on the 

 A<laptive Coloration of Molluska, " Proc. Boston Soc. of Nat. Hist.," 

 vol. xiv, April, 1871. 



