CiiAP. IX.J MOLLUSKS AND ANXELIDS. 317 



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

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

 shell depending on its manner of growth. The amount 

 of light seems to a certain extent to be influential ; for 

 although, as repeatedly stated by Mr. Gwyn Jeflreys, 

 the shells of some species living at a profound depth 

 are brightly colored, yet we generally see the lower sur- 

 faces and the parts covered by the mantle less highly 

 colored than the upper and exposed surfaces.' In some 

 cases, as Avith shells living among corals or brightly-tinted 

 sea-weeds, the bright colors may serve as a protection. 

 But many of the niidibranch mollusca, or sea-slugs, are 

 as beautifully colored as any shells, as may be seen in 

 Messrs. Alder and Hancock's magnificent work ; and from 

 information kindly given me by Mr. Hancock, it is ex- 

 tremely doubtful whether these colors visually serve as a 

 protection. With some species this may be the case, as 

 with one which lives on the green leaves of algse, 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 re- 

 cesses. So that, with these niidibranch mollusks, color 

 apparently does not stand in any close relation to the na- 

 ture of the places which they inhabit. 



These naked sea-slugs are hermaphrodites, yet they 

 pair together, as do land-snails, many of which have ex- 

 tremely pretty shells. It is conceivable that two hermaph- 

 rodites, attracted by each other's greater beauty, might 

 unite and leave offspring which would inherit their par- 

 ents' greater beauty. But with such lowly-organized 



^ I have given (' Gcolog. Observations on Yolcanic Islands,' 18-14, p. 

 C3) a curious instance of the influence of light on the colors of a fron- 

 descent incrustation, deposited by the surf on the coast-rocks of Ascen- 

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



