[Illustration: FIG. 48.--Two stages in the development of _Loligo vulgaris_ Lam.: _a_{1}_, _a_{1}_, first, and _a_{2}_, _a_{2}_, 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. =The Reproductive System.=--Broadly speaking, the complicated arrangements which are found in Mollusca resolve themselves into modifications of three important factors:-- (_a_) The _gonads_ or _germ-glands_, in which are developed the ova and the spermatozoa. These glands are generally known as the _ovary_ in the female, the sperm-gland or _testis_ in the male.