Chap. IX. Molluscs. 263 



Mr. Lonsdale, informs me that he placed a pair of land-snails, 

 (Helix pomatia), one of which was weakly, into a small and ill- 

 provided garden. After a short time the strong and healthy 

 individual disappeared, and was traced by its track of slime 

 over a wall into an adjoining well-stocked garden. Mr. 

 Lonsdale concluded that it had deserted its sickly mate; but 

 after an absence of twenty-four hours it returned, and apparently 

 communicated the result of its successful exploration, for both 

 then started along the same track and disappeared over the 

 wall. 



Even in the highest class of the Mollusca, the Cephalopoda or 

 cuttlefishes, in which the sexes are separate, secondary sexual 

 characters of the present kind do not, as far as I can discover, 

 occur. This is a surprising circumstance, as these animals 

 possess highly-developed sense-organs and have considerable 

 mental powers, as will be admitted by every one who has watched 

 their artful endeavours to escape from an enemy. 3 Certain 

 Cephalopoda, however, are characterised by one extraordinary 

 sexual character, namely, that the male element collects within 

 one of the arms or tentacles, which is then cast off, and clinging 

 by its sucking-discs to the female, lives for a time an independent 

 life. So completely does the cast-off arm resemble a separate 

 animal, that it was described by Cuvier as a parasitic worm 

 under the name of Hectocoiyle. But this marvellous structure 

 may be classed as a primary rather than as a secondary sexual 

 character. 



Although with the Mollusca sexual selection does not seem to 

 have come into play; yet many univalve and bivalve shells, 

 such as volutes, cones, scallops, &c, are beautifully coloured 

 and shaped. The colours 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 manner 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 

 coloured, yet we generally see the lower -surfaces, as well as the 

 parts covered by the mantle, less highly-coloured than the 

 upper and exposed surfaces. 4 In some cases, as with shells 



3 See, for instance, the account, influence of light on the colours of 

 which I have given in my 'Journal a trordescent incrustation, de- 

 af Researches,' 1845, p. 7. posited by the surf on the coast- 



4 I have given (' Geolog. Obser- rocks of Ascension, and formed by 

 rations on Volcanic Is'ands,' 1844, the solution of triturated sea-uhcsls 

 p. 53) a curious instance of the 



