Coelom, Renal and Reproductive Organs.—The coelom is represented in most Mollusca by the pericardium and (when pei sistent) the cavity of the gonad. In the Cephalopoda there is a capacious gonadial portion of the coelom from the wall of which the ovary and testis are developed. In this class and certain Am phineura (the Aplacophora) the pericardium and gonadial coelom communicate with each other, but in all other molluscs the two portions of the coelom are separate. It is however to be inferred that the primitive connection between pericardium and gonad occurred in the ancestors of such molluscs, as in the more primi tive members of the Gastropoda and Lamellibranchia the gonad opens into the duct connecting the kidney and the pericardium. It is not certain if the renal and generative ducts in all the groups of Mollusca are always true coelomoducts (i.e., evaginations of the coelomic cavity). The kidneys open internally into the peri cardium in all the classes except in Nautilus and the Scaphopoda. Whether this connection actually indicates that the kidneys are in all cases coelomic outgrowths is not certain. True nephridia only occur in larval Gastropoda and Lamellibranchia. In certain members of these two classes which have been carefully studied the adult kidneys 'are certainly mesodermal in origin and may be regarded as coelomoducts. In the Aplacophora, the archaic Gas tropoda and the Scaphopoda there are no separate genital ducts, the ova and spermatozoa being discharged through the kidneys. This is likewise found in certain Lamellibranchia, but the genital conduit is gradually shif tea towards the external end of the kidney and eventually acquires a separate opening. In the Neritacea among the Aspidobranchiate Gastropoda and the rest of that class in such forms as have been specially studied there is a separate genital duct formed from the originally left-hand kidney. In the Cephalopoda and Polyplacophora the gonaducts and kidneys are entirely separate.
The Nervous System.—The nervous system consists of ganglionic centres, nerve cords and sense-organs. The term "cen tral nervous system" is not usually employed in connection with the nervous organization of the Mollusca; but there is in fact a :entral group of ganglia connected by commissures and a periph Tal system of nerves, localized ganglia and end-organs. The gen !ral plan of the system has been already indicated. The chief areas )f the body and organs are innervated as follows. The head and is sense-organs are supplied with nerves from cerebral ganglia sit Iated on the dorsal side of the circumoesophageal ring. The nantle and foot are innervated from pallial and pedal ganglia 7espectively from which the main pallial and pedal cords origi late. The viscera receive their nerve supply from the labial com nissure (the ventral part of the circumoesophageal ring) through stomato-gastric system and also from a visceral commissure beset with ganglia which arises from the pallial ganglia. In prim itive Mollusca the various ganglia are diffuse and the commissures joining them are long. In Gastropoda and Cephalopoda there is I marked tendency for the ganglia to be concentrated around the oesophagus and fused in a solid ring. The term "central nervous system" then becomes highly suitable. Whether this concentra tion has any functional significance is uncertain. It may be re sponsible for the highly co-ordinated movements performed by the Cephalopoda, but as far as nervous efficiency is concerned there does not seem much difference between those Gastropoda in which the ganglia are concentrated and those in which they are diffuse.
Reproduction.—The sexes are separate in most Mollusca and hermaphroditism is usually found in specialized forms. Sexual
dimorphism is not marked. Copulation only occurs in the Cephalo poda and in certain Gastropoda in which a penis is found, but in some Lamellibranchia in which there is no copulatory organ fertil ization is nevertheless internal. The eggs are laid separately in the Amphineura, Lamellibranchia, Scaphopoda and more archaic Gas tropoda ; in most aquatic Gastropoda and in the Cephalopoda they are deposited in masses which through the development of spe cial capsules and supporting structures may become very complex. In a few Gastropoda, Lamellibranchia and Amphineura the eggs are incubated in the maternal body ; but truly viviparous forms are rare. Parthenogenesis has been recorded in a single species of Gastropoda, Paludestrina jenkinsi. The development of the fer tilized egg is described in detail under the separate classes (Gastropoda, etc., qq.v.).
Bionomics.—The most primitive living molluscs are aquatic and the earliest fossil representatives (Lower Cambrian Gastro poda and Lamellibranchia) seem to have lived in the sea. The greater part of the modern Mollusca have retained this character. During the Devonian period they began to populate fresh water and in the Carboniferous they invaded the land. From that time onwards the population of freshwater and land has continued, that of land with increasing rapidity and evolutionary diversification, that of fresh water more slowly and with less variety of form and habit. The relative numbers of marine, fresh water and terres trial forms may be gauged by the following list which gives the number of species in the British molluscan fauna as far as they can be thus classified. Marine 714, Fresh water 69, Land 91.
The Amphineura, Scaphopoda and Cephalopoda are exclusively marine in distribution and do not tolerate any reduction of the normal salinity of sea water. The freshwater fauna is made up principally of two groups—certain Tenioglossa (Vivipara, Hy drobia, Melania, Ampullarza) and Basommatophora (Limnea and Planorbis) among the Gastropoda and the Unionidae, Cyrenidae, etc., among the Lamellibranchia. The large terrestrial fauna is mainly drawn from a single suborder (the Stylommatophora) and a few families (e.g., Helicinidae, Cyclophoridae, Auriculidae, etc.) of other Gastropoda. In other words, the population of fresh water is the achievement of a restricted group of Gastropoda and Lamellibranchia and the highly successful invasion of land has been carried out by the Gastropoda alone. As far as the Lamelli branchia are concerned the failure to populate land is readily ex plained, as their mode of nutrition has become highly specialized and entirely depends on water-borne particles and organisms. The absence of Cephalopoda from land and fresh water is not so easily explained. The Dibranchia are specialized for rapid swimming, so that the reason for their restriction to water is obvious. The littoral Octopoda, which live mainly on or just off the sea bottom, can move over the latter with con siderable agility so that locomotor special ization cannot be invoked to account for their absence from the land. In all likeli hood the marked participation of the man tle in the locomotor function and its de velopment as a muscular pumping organ in relation to respiration has deprived it of that capacity for being converted into an organ for breathing air characteristic of terrestrial Gastropoda. As for their failure to colonize fresh water there is no satis factory explanation beyond the obvious suggestion that they may be physiologically intolerant of fresh and brackish water.