=Spermatozoa.=--The shape of the spermatozoa and of the ova in Mollusca is of the usual type. In _Paludina Ampullaria_, and certain species of _Murex_ two types of spermatozoa occur, one hair-like, the other worm-like, three times as long as the former, and not tapering at one end. The former type alone take part in fertilisation, and penetrate the ovum. It has been suggested that these worm-like spermatozoa are a kind of incipient ova, and indicate a possible stage in commencing hermaphroditism. And, since the nearest allies of the Prosobranchiata (in which these types occur) are hermaphrodite (_i.e._ the Opisthobranchiata and Pulmonata), it is not unreasonable to suppose that the Prosobranchiata should show some tendency towards hermaphroditism in their genital glands.[256] =Cephalopoda.=--The special characteristic of the reproductive organs in female Cephalopoda is the development of various glands, some of considerable size, in connexion with the ovary and oviduct. _Sepia_, _Loligo_, and _Sepiola_ are furnished with two large _nidamental glands_, which open into the mantle cavity independently of the oviduct. Their purpose is to produce a viscid mucus, which envelops the ova at the moment of their emission and eventually hardens into the egg-capsules. A pair of accessory nidamental glands occur in _Sepia_, as well as a pair of smaller glands situated on the oviduct itself. In many of the male Cephalopoda the vas deferens is long and dilated at its outer end into a glandular reservoir, within which are formed the spermatophores, or narrow cylindrical packets which contain a very large number of spermatozoa. When charged, the spermatophores pass into what is known as Needham’s sac, where they remain until required for use. These spermatophores are a very characteristic part of the reproductive arrangements in the Cephalopoda. The male of _Sepia_ has been noticed to deposit them, during union, upon the buccal membrane of the female. During the emission of the ova by the female, the spermatophores, apparently through the agency of a kind of spring contained at one end, burst, and scatter the spermatozoa over the ova. =The Hectocotylus Arm.=--Perhaps the most remarkable feature in the sexual relations of all the Mollusca is the so-called _hectocotylus_ of the Cephalopoda. In the great majority of the male Cephalopoda, one of the ‘arms,’ which is modified for the purpose in various ways and to a greater or less extent, becomes charged with spermatophores, and sometimes, during union, becomes detached and remains within the mantle of the female, preserving for some considerable time its power of movement.