Fig. 48.—Two stages in the development of Loligo vulgaris Lam.: a1, a1, first, and a2, a2, second pairs of arms; br, branchiae, seen through m, mantle; e, e, eyes; fi, fins; fu, funnel; v.s, vitelline sac. (After Kowalewsky.)
In some cases—e.g. certain Pelecypoda—what is practically a third type occurs. The animal is hermaphrodite, but the male and female elements are not developed simultaneously, i.e. the same individual is at one time female, at another male.
1. The sexes are separate in
- All Cephalopoda.
- Gasteropoda Amphineura (except Neomeniidae).
- Gasteropoda Prosobranchiata (except Valvata and some species of Marsenia).
- Scaphopoda.
- Many Pelecypoda.
2. The sexes are united in
- Gasteropoda Opisthobranchiata.
- Gasteropoda Pulmonata.
- Certain Pelecypoda.[254]
In the dioecious Mollusca, sexual union is the rule, but is by no means universal. In some instances,—e.g. Vermetus, Magilus, Patella, Haliotis, Crepidula, Chiton, the Scaphopoda—the form and habits of the animal do not admit of it; in others (many Trochus) a male copulative organ is wanting. When this is the case, the male scatters the spermatozoa freely; the majority must perish, but some will be carried by currents in the direction of the female.
When the sexes are separate, the female is frequently larger than the male. This is markedly the case in Littorina, Buccinum, and all the Cephalopoda; in Argonauta the difference is extreme, the male not being more than ¼ the size of the female.
Those hermaphrodite Mollusca which are capable of sexual union (Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently divided into two sections, according as (1) there are separate orifices for the male and female organs, or (2) one orifice serves for both. To the former section (Digonopora[2]) belong the Limnaeidae, Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including all the Pteropoda; to the latter (Monogonopora[255]) nearly all the Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata. In the latter case during union, mutual impregnation takes place, and each of the two individuals concerned has been observed (compare p. [42]) to deposit eggs. In the former, however, no such reciprocal act can take place, but the same individual can play the part of male to one and female to another, and we sometimes find a string of Limnaea thus united, each being at once male and female to its two adjacent neighbours.