The respiratory organs are ctenidia, or gills, external processes of the body. In some instances, Pteropoda, a few Gastropoda, respiration is carried on entirely by the surface of the skin. There can be little doubt, however, that such forms have lost the gills. A ctenidium consists essentially of an axis containing an afferent and efferent blood-vessel and giving support on either side to a series of vascular lamellae or processes, the surface of which is ciliated except in Cephalopoda. The ctenidial axis remains either free, e. g. Nautilus; or partly free, partly attached to the side of the sub-pallial space; or attached throughout its whole length, e. g. Dibranchiate Cephalopoda. The respiratory processes it bears generally assume a most complicated structure in Lamellibranchiata. The ctenidia are always lodged in the sub-pallial space, which may be much enlarged in the region where they lie forming a branchial cavity. In the Polyplacophora there are a number of them, but in all other Mollusca they are typically two, one on each side of the body. In the majority of Gastropoda the primitive left ctenidium is aborted.
Occasionally both are aborted, and then respiration is carried on either by secondarily developed vascular ridges lodged in the sub-pallial space, as in the Limpet, or by the part of the mantle which forms the roof of the branchial cavity, as in Pulmonate Gastropoda, or by the surface of the body.
The nervous system consists typically of a pair of cerebral ganglia, placed above the oesophagus, and two pair, pedal and pleural, placed below it. The two members of each pair of ganglia - cerebral, pedal and pleural - are united to one another by nerve-commissures: the cerebral to the pedal and pleural, as well as the pedal to the pleural, by nerve-connectives. Commissures and connectives vary in length, and consequently there is not only a greater or less degree, but also a varying mode of concentration of the ganglia among themselves. The cerebral ganglia supply the head, the organs of touch, the otocysts (except in Lamellibranchiata?), and the cephalic eyes as well as the pharynx. The pedal ganglia supply the foot. The pedal nerves are sometimes connected by a series of transverse commissures. Each pleural ganglion is continued backwards into a nerve which unites posteriorly below, rarely above the intestine, with its fellow, forming a visceral loop. This loop typically carries a pair of visceral ganglia, and a posterior abdominal ganglion. The pleuro-visceral system thus constituted supplies the viscera and the walls of the body exclusive of the foot, and special ganglia are not infrequently developed on its nerves.
In Fissurella and Haliotis (Gastropoda Anisopleura), the pedal ganglia are replaced by long cords with an outer coat of ganglion cells, extending down the foot and connected by transverse fibrous commissures. The pleural ganglia are only incompletely differentiated from the anterior ends of these cords. In Chiton and its allies there are pleural as well as pedal cords both connected with a cerebral ring, all alike invested with ganglion cells. The cords are derived in Chiton, as are the ganglia in nearly all Mollusca, from the epiblast. It is possible that these forms of nervous system represent a primitive state, and that the concentration of ganglion cells into ganglia is a later state. A pair of buccal ganglia are developed in the Glossophora in connection with the buccal mass upon which they lie. They supply the salivary glands, oesophagus and radula, and are connected to the cerebral ganglia; but it appears that in Fissurella, Haliotis and Turbo they are really connected to the pleuro-pedal centres, their connective only traversing the cerebral ganglia (Haller).
Special sense-cells provided with immobile sense-hairs have been found in the epidermis of Gastropoda and Lamellibranehiata. A special patch of ciliated epithelium lies close to the base of the gills. A ganglion in connection with a nerve derived from one of the visceral ganglia or from the visceral loop underlies this patch. The whole constitutes the osphradium, an organ which probably detects changes in the water passing over the gills. The osphradium occurs in all groups of Mollusca except Scaphopoda. An olfactory (?) pit is found near the eye in Cephalopoda. Cephalic eyes are absent in Scaphopoda and Lamellibranehiata, and are rudimentary in Pteropoda if present. Eyes of a simple or complex structure are scattered along the mantle-margin in many Lamellibranehiata. In two instances (Pecten, Spondylus) these mantle-eyes bear a singular resemblance to the Vertebrate eye: the visual rods are turned away from the light, and there is a cellular lens derived however from the mesoblast. A cellular lens and visual rods, turned as in Pecten and Spondylus, are again met with in the dorsal eyes of certain species of the Gastropod genus Onchidium. These three instances are, so far as is known, the sole exceptions to the rule that the visual rods of Non-Vertebrates are directed towards the light A cellular lens also only recurs among Non-Vertebrates in the Medusa Charybdaea1. The otocysts nearly always lie close to the pedal ganglia.
They generally arise from the epiblast as pits which close forming a vesicle, sometimes as solid ingrowths. They contain one or more calcareous otoliths, and are lined by an epithelium which is generally ciliated and occasionally provided with sense-hairs.
The digestive tract is composed of a stomodaeum, a mesenteron, and in some instances at least a proctodaeum. The stomodaeum in the Glossophora is muscular, has appended salivary glands, and contains an organ known as radula, composed of a chitinous membrane bearing chitinoid teeth, developed within a sac (radular or odontophore sac), and growing throughout life. It is borne in turn upon a subradular membrane and cartilages, the latter always, the former sometimes, moved by muscles, the whole constituting the odontophore. The shape and arrangement of the teeth vary much; for development, etc, see p. 115. The mesenteron is of some length. It is rarely straight, and its disposition is affected by the mode of growth of the visceral dome. It makes a single simple bend upon itself in Cephalopoda and Pteropoda, the concavity of the bend being turned to the pedal ganglia. In other Mollusca it is more or less coiled. A stomachal widening can generally be distinguished. A liver is always present in the shape of either simple caeca appended to the stomach or of a large gland, paired except in Cephalopoda. The cells of the liver follicles are of three kinds - granule cells, ferment and lime-producing cells. The first-named are always present.