| OCR Text |
Show 324 SEXUAL SELECTION. PART II. The sub-kingdom of the Mollusca.-Thro~ghout this o-reat division (taken in its largest acceptatiOn) of the ~nimal kingdom, secondary sexual characters, ~uch as we are here considering, never, as far as I can discover, . N . ld they be expected in the three lowest occur. or cou . . p I nd Brachio-classes namely in the Asmd1ans, o yzoa, a pods (~onstituting the Molluscoida of Huxley), for most of these animals are permanently affixe.d ~o .a support or have their sexes united in the same mdrvidual. In the Lamellibranchiata, or bivalve shells, hermaphroditism is not rare. In the next higher class of the Gasteropoda or marine univalve shells, the sexes are either united or separate. But in this la~ter case .the males never possess special organs for findmg, securmg, or charming the females, or for fighting with other males. The sole external difference between the sex:s consists as I am informed by Mr. Gwyn Jeffreys, 111 the sh~ll sometimes differing a little in form ; ~or instance, the shell of the male periwinkle (L_ittonna Zittorea) is narrower and has a more elongate~ spue tha?that of the female. But differences of this nature, It may be presumed, are directly connected with the act of reproduction or with the development of the ova. The Gasteropoda, though capable of locomotion and furnished with imperfect eyes, do not appear to be endowed with sufficient mental powers for the members of the same sex to struggle together in rivalry, and thus to acquire secondary sexual characters. Nevertheless with the pulmoniferous gasteropods, or landshells, the pairing is preceded by courtship ; for the~e animals, though hermaphrodites, are compelled by the~r structure to pair together. Agassiz remarks/ "Qm,, conque a eu I' occasion d'observer les amours des lima- 1 'De l'Espece et de Ia Class.' &c., 1869, p. 106. CJJAP. IX. MOLLUSCS. 325 "<;ons, ne saurait mett.re en doute la seduction deployee "dans les mouvements et les allures qui preparent et "accomplissent le double embrassement de ces her" maphrodites." These animals appear also susceptible of some degree of permanent attachment: an accurate obsener, 1\'Ir. Lonsdale, informs me that he placed a pair of land-shells (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, namely the Cephalopoda or cuttle-fishes, in which the sexes are separate, secondary sexual characters of the kind which we are here considering, 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.2 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 2 See, for instance, the account which I have given in my 'Joumal of Researches,' 1845, p. 7. |
