Sex [electronic resource] . Fio. 17.—Female of Paper Nautilus—Argonauta argo—withits brood-ohambei shell, enveloped by the expanded eruisof two of the arms. the essential gonads, and are therefore bereftof that useful criterion of a secondary sex-character which has been discovered invertebrates. Let us consider, then, a few striking sex-differences in their bearing upon Tand-erstheory. The female paper-nautilus, or ar-gonaut, is very different from the male. Sheis much larger, she has two arms peculiarlymodified to secrete a unique shell, not homo- 90 SEX logous with other cephalopod shells,
Sex [electronic resource] . Fio. 17.—Female of Paper Nautilus—Argonauta argo—withits brood-ohambei shell, enveloped by the expanded eruisof two of the arms. the essential gonads, and are therefore bereftof that useful criterion of a secondary sex-character which has been discovered invertebrates. Let us consider, then, a few striking sex-differences in their bearing upon Tand-erstheory. The female paper-nautilus, or ar-gonaut, is very different from the male. Sheis much larger, she has two arms peculiarlymodified to secrete a unique shell, not homo- 90 SEX logous with other cephalopod shells, whichis used as a brood-chamber for the developingova (see Fig. 17). The small male has nosuch shell and no such modification of twoof the arms. When he is sexually mature, oneof his arms becomes laden with sperm-packetsand is discharged as a hectocotylus intothe mantle cavity of the female (see Fig. 18).These are familiar facts, but we do not. Fig. 18.—The minute male of the Paper Nautilus—Argonautaargo—showing the mouth surrounded by eight arms,one of which has been hectoeotylised, i. e. muchspecialised for reproduction. The locomotor funnel andthe mantle opening may be seen to the left, one of thoeyes to the right. Twice natural size. (After Muller.) know of any evidence for supposing that theancestors of the argonaut ever had an ex-ternal shell of the argonaut type or modifiedarms such as the female now shows. Thereis no hint of such a thing. Moreover, theshell is not for living in, but for the protectionof the eggs; it is a cradle, not a house, and it THEORY OF SEX-DIMORPHISM 91 has no meaning except in the female. Unlessindeed one supposes that both sexes used tocarry the eggs, or that the ancestors ofArgonauta were hermaphrodites—but for anysuch hypotheses, there are no bases. Let us take one of those very interestingcases where the female has something definiteand positive which the male has not—thefrog Not
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