These accessory organs are most complicated in Pteropoda and hermaphrodite Gastropoda. Impregnation of the ovum may take place externally to the organism, as in Cephalopoda (? all), Scaphopoda, Chiton and Zygobranchia among Gastropoda, and most Lamellibranchiata; within the genital duct in Pteropoda and most Gastropoda. The spermatozoa are contained in spermatophores in Cephalopoda and pulmonate Gastropoda. The ova vary much in the amount of food-yolk they contain, and therefore in their mode of segmentation. They are largest in Cephalopoda where the only part of the ovum that segments is, as in the Sauropsida, a disc of protoplasm. The ovum is devoid of primary egg-membranes in Gastropoda and Cephalopoda.
The Mollusca with the exception of Cephalopoda pass through a typical larval development, in two stages - a Trochosphere and a Veliger stage. The Molluscan Trochosphere has a ventral mouth, bent intestine, and ventral or terminal anus. It has a praeoral lobe, variable in size, but encircled by a band of cilia - the velum - at its base, with in some cases a postoral band of fine cilia (Mc' Murrich, A. N. H. (5), xvi. 1885). It is especially characterised by a ventral projection between the mouth and anus, the rudimentary foot, and by a dorsally placed depression lined by epiblast - the shell-gland. It passes into the Veliger stage in which the velum enlarges, and in the larvae of Pteropoda and Gastropoda is drawn out into at least two lobes, one right, the other left: the shell gland may secrete a chitinous plug, but it flattens out (except in a few kidney in certain regions of the integument and walls of the pericardium of some Gastropoda Opisthobranchia; Trinchese, Arch. Biol. Ital. iv. 1884. instances) and forms an area upon which the shell begins to develope, while its edges thicken and extend, and eventually form the edges of the mantle: the foot also enlarges in a manner characteristic of the group to which the Veliger belongs.
It generally bears an operculum upon the dorsal aspect of its hinder extremity in Pteropoda and Gastropoda, whilst in Lamellibranchiata a byssus gland appears as an epiblastic involution at its edge. Both operculum and byssus gland may persist or disappear. Individual peculiarities occur in different groups - e. g. Chiton, Dentalium, and many Lamellibranchiata possess a flagellum or tuft of cilia projecting from the centre of the velar area; a posterior tuft of cilia may also occur. The normal course of development may be abbreviated or modified in freshwater, terrestrial or viviparous forms. The Cephalopoda have a modified development owing to the presence of food-yolk. The yolk-sac appears to occupy the position of the foot, and the velum is either not represented at all, or only in a very rudimentary manner. The edge of the blastoderm is ciliated in some instances. The shell-sac is formed by two dorsal folds, and does not appear to be homologous with the shell-gland which is therefore not represented.
The great majority of Mollusca are water-breathers, and marine in habitat: some Gastropoda and Lamellibranchiata are fluviatile and lacustrine: and a few Gastropoda are terrestrial and air-breathers. The remains of Cephalopoda, Pteropoda, Gastropoda and Lamellibranchiata occur in the lower Silurian strata.
The Mollusca may be grouped in two main divisions, the Glossophora and Lipocephala of Ray Lankester, sometimes also termed Cephalophora and Acephala. The division Lipocephala comprises the single class Lamellibranchiata; the division Glossophora the four classes Cephalopoda, Scaphopoda, Pteropoda and Gastropoda.
The Pteropoda and Cephalopoda are associated together by Professor Lankester as Cephalopoda - the class here called Cephalopoda receiving the name Siphonopoda. This arrangement has not been followed because the Pteropoda seem to be a group which are really asymmetrical and have acquired an external symmetry. External asymmetry is preserved in the Limacinidae with their spirally twisted visceral dome and shell; and in the spiral shell of the larval Cymbulia and Tiedetnannia. The occasional presence of an operculum in the adult and larva; the asymmetry observable in the osphradium, and nephridium, and in the position of the anus; the structure of the genital organs are points in which the members of the class strongly resemble the Gastropoda, as they do also in development. The cephalic appendages of the Pteropoda have probably nothing to do with the arms of a Cephalopod. The latter are supplied by the pedal nerve, whilst the buccal cones of Clio, the only Pteropod very accurately monographed, are supplied by the cerebral ganglia. It is possible that the arms of the Cephalopod represent the epi-podium. In this case they may be compared with the epipodia of Thecosomatous Pteropoda which have grown round the mouth.
And in the three lobes of the median foot, as seen in Clio, structures are present which might well represent the lobes and valve of the funnel in a Cephalopod. But the last mentioned are points on which there is great difference of opinion. See Grobben, 'Morphologische Studien iiber den Harn und Geschlechtsapparat, etc., der Cephalopoden' Arb. Zool. Inst. Wien, v. 1884, pp. 44-70. for a recent discussion of them and of the affinities of Cephalopoda.