The marine Mollusca have a world-wide distribution as seden tary, swimming and floating animals and they are found at con siderable depths in the sea, a few being found at a depth of over 2,900 fathoms. Certain groups are specialized for living perma nently at great depths, e.g., the Cirroteuthidae among the Dibran chiate Cephalopoda and the Septibranchia among the Lamelli branchia. The Pteropods, Heteropods and a few other Gastro poda, and the smaller branchiiform Cephalopoda are adapted in adult life as plankton organisms. By far the greater part of ma rine molluscs are sedentary and live on the bottom in relatively shallow water. Land and fresh water molluscs are likewise of almost universal distribution. The terrestrial forms are usually limited in their range by the requirement of a measure of humid ity, shelter from excessive heat and light and of a certain amount of lime in the soil.
From the bionomic point of view the Cephalopoda stand apart from the rest of the Mollusca. Nearly all the living forms are highly mobile and vigorous animals, sometimes of large size and predatory habits. In these respects they are unique among inverte brate animals and afford the closest parallel to the vertebrates in organization and activity. The rest of the phylum are principally sluggish animals usually of small size and retiring habits. The greater number are vegetarian in diet. Certain Gastropoda are, however, carnivorous and, though their prey consists of either sessile animals such as Hydroids and sponges or slow-moving ani mals such as other molluscs, they are more vigorous and aggres sive. The majority are, however, encumbered by their shell, and rely on passive defence rather than attack or flight in dealing with their enemies.
The Scaphopoda and Lamellibranchia live a specialized life buried in sand or mud. To this sort of life the lamellibranchs are particularly well adapted. They can live for a long time buried below the surface; but, as is pointed out in the article on this group (q.v.), this mode of life is not necessarily a sluggish and inactive one. Not only is a considerable amount of energy required to burrow in a semi-solid medium, but also those Lamellibranchia which live on open beaches must be always under the necessity of regulating their position during rough weather when the surface layers of sand are in continual motion. The Amphineura are mostly sluggish, semi-adherent forms. The Gastropoda are more varied in their habits and include active carnivorous families, adherent and floating forms. Others are climbers, burrowers and swimmers. Both the Gastropoda and Lamellibranchia include commensal and parasitic forms as well as genera which bore into solid substances (rock, coral, etc.).
The Mollusca are on the whole short-lived. Most pulmonate Gastropoda are biannual, though some large Helicidae live as long as seven years. The marine Streptoneura live for several years
but the Nudibranchs and Tectibranchs seem to be "annuals." The Cephalopoda, as far as is known at present, seem to have an average life of four to five years. Certain Lamellibranchia are known to live as long as 20 years.
Interrelationships of the Various Classes.—Although it has been urged that the Solenogastres are not molluscs and should be placed in a different phylum and although Von Ihering has tried to prove that the Mollusca as at present constituted are not a homogeneous group, there seem to be no adequate grounds for doubting Pelseneer's dictum (1906) that the internal organization of the members of the phylum is remarkably uniform. In spite of very marked plasticity in external parts, the fundamental struc tural plan as sketched above remains singularly constant and unal tered. It is also significant that the larger classes all tend to show similar types of modification. To cite a single instance, the shell in all Mollusca save the Scaphopoda exhibits a constantly recur ring tendency to become covered by the mantle and atrophied.
Nevertheless the relationship between the various classes is by no means clear. They each represent distinct evolutionary tenden cies and it is by no means certain how far we can state that cer tain classes are more closely related than others. Grobben, for example, united the Gastropoda, Scaphopoda and Lamellibranchia into a grade, the Prorhipidoglossomorpha, distinct from the Amphineura and Cephalopoda. This association is up to a cer tain point justifiable. But it fails to express (a) the peculiar posi tion of the Lamellibranchia which must have diverged along their distinctive evolutionary path at an early date in the history of the phylum and (b) the considerable resemblances between the Gastropoda and Cephalopoda. It is a useful working hypothesis to assume that there are three distinct elements in the phylum .. (I) the Amphineura representing the most primitive age of devel opment, (2) the Cephalopoda and (3) the Gastropoda, Lamelli branchia and Scaphopoda. It is true that the Amphineura (par ticularly the Chitons) have a simpler organization than the other Mollusca and may be considered to be the most primitive mem bers of the phylum, though it is peculiar that they are not found as fossils until the Silurian (Ordovician?), whereas the other chief classes are recognizable in the Cambrian. Nevertheless it is not easy to employ the total complex of characters which each class represents, as an index of relationship or to decide with confidence which of those characters are actually primitive or how far re semblances have been brought about by convergence. For that reason it seems desirable to stress the individual and peculiar des tiny of each class rather than insist on any grouping of the classes.