The Cambridge natural history . inospira, and Calcarclla. Cephalopoda.—The endjryonic development of tlie Cephalopodais entirely distinct from that of all other Mollusca. Thesegmentation of the vitellusis partial, and the embryo isfurnished with a vitellinesac, which is very large inthe majority of cases (). There is no free-swimming stage, but theembryo emerges from theegg fully developed. Differences of Sex.—Inthe Mollusca there are twomain types of sexual differ-ence : (i) sexes separate (dioe-cious type), (ii) sexes unitedin the same individual (her-Viaphrodite type). In some cases—e
The Cambridge natural history . inospira, and Calcarclla. Cephalopoda.—The endjryonic development of tlie Cephalopodais entirely distinct from that of all other Mollusca. Thesegmentation of the vitellusis partial, and the embryo isfurnished with a vitellinesac, which is very large inthe majority of cases (). There is no free-swimming stage, but theembryo emerges from theegg fully developed. Differences of Sex.—Inthe Mollusca there are twomain types of sexual differ-ence : (i) sexes separate (dioe-cious type), (ii) sexes unitedin the same individual (her-Viaphrodite type). In some cases— certain Ielecypoda—what is practicallya third type occurs. The animal is hermaphrodite, but the maleand female elements are not developed simultaneously, thesame individual is at one time female, at another The sexes are separate in All Cephcdopoda. Gasteropoda Amphineura (except Neomeiiiidae). Gasteropodct Prosohrancliiatct (except Vcdvata and somespecies of Marsenia). tSrnpli opoda. I\Iany Fig. 48.—Two stages in the development ofLoligo vulgaris Lam.: aj, «i, first, and «o, «2,second pairs of arms ; 6r, branchiae, seenthrougli m, mantle ; e, e, eyes ; fi, tins ; fv,funnel ; , vitelline sac. (After Kowal-ewsky.) 134 TYPES OF SEXUAL DIFFERENCE chap. 2. The sexes are united in Gasteropoda Pelecypoda} In the dioecious MoUusca, sexual union is the rule, but is byno means universal. In some instances,— Vermetus, Magilus,Patella, Haliotis, Crepidula, Chiton, the Scaphopoda—the formand habits of the animal do not admit of it; in others (manyTrochus) a male copulative organ is wanting. When this is thecase, the male scatters the spermatozoa freely; the majority mustperish, but some will be carried by currents in the direction of thefemale. When the sexes are separate, the female is frequently largerthan the male. This is markedly the case in Littorina, Buccinum,and all the Cephalopo
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
