The nervous system in the mollusca is developed in accordance with two distinct types. In the lowest group of this sub-kingdom (the molluscoicls), there is only a single ganglion with afferent and efferent fibers radiating in every direction; while in the higher groups there are several ganglia lying somewhat irregularly in different parts of the body, and communicating by nervous threads with a larger mass placed in the head, or in the neighborhood of the (esophagus. This mass consists of several ganglia, which from their position are termed supracesophageal, and is united by filaments with other ganglia lying below the oesophagus, so as to form a ring or collar around that organ. The supracesophageal ganglia furnish the nerves to the special organs of the senses. Most of the molluscs possess special organs of touch in the form of lips or of special lobes around the mouth; of tentacles or arms upon the head, or of cirrhi upon other parts of the body; and in addition to these special organs, the skin appears to possess considera ble sensibility. When tentacles are present, they are either two or four in number; and they can be protruded and retracted at pleasure, as every one must have noticed in the case of these organs (popularly known as horns) in the snail. Organs of sight are not universally present. In many mollusca there is only a single rudimentary eye, while in others there is a large number of imperfect eyes (termed ocelli), which do not of necessity lie in the region of the head. In the higher mollusca there are two eyes, sometimes placed directly on the head, and sometimes on the tentacles; and in the highest group (the cephalopods) the eyes are as fully developed as in fishes.
Organs of hearing, in a simple form, are almost always present. They usually consist of round vesicles in the neighborhood of the oesophageal ring, from which they receive a nervous filament. They contain a clear fluid and a small concretion of carbonate of lime, which is sometimes roundish, and sometimes of a crystalline form, and is in a perpetual state of vibration, in consequence of ciliary action in the interior of the vesicle. Whether there are any special organs of smell and taste in the molluscs is still undecided.
The organs of vegetative life (of circulation, etc.) are much more fully of ' developed in the molluscs than those animal life. The alimentary canal, which pre sents almost every variety of form from a simple cavity to a complicated intestine, is always provided with two distinct openings, a mouth and an anus, the latter being often situated (as in the gasteropoda and pteropoda) on the right side of the anterior part of the body. The liver is always present, existing in a mere rudimentary form in the polyzoa, constituting a large part of the body in the acephalous bivalve mollusca (as the mussel and cockle), and a still larger part in the gasteropoda (as the snail), while in the cephalopoda it is constructed upon nearly the same plan as in fishes. Other secret ing organs, such as salivary glands, pancreas, and urinary organs, are also present in the more highly developed molluscs.
The circulation of the blood is effected (except in the polyzoa) by means of a distinct heart, which usually communicates with a regular, closed vascular system; but in some cases the venous System Hunerfeet, and blood which hos been transmitted by the arteries to the system in general is not confined within distinct vessels, but meanders through sinuses or passages excavated in the tissues, and through them it reaches the respiratory apparatus, whence it is transmitted by closed vessels (veins) to the heart. The blood is nearly colorless (sometimes of light blue or green tint), and contains but Tew floating corpuscles. In all but the very lowest mollusea there is a distinct respira
tory apparatus, which, excepting in the ease of the terrestrial gasteropoda (as, for example, the snail), is constructed with a view to aquatic respiration, and is composed of branchia, or gills. These branchite usually consist of a series of membranous plates arranged like the leaves of a book or the teeth of a comb), over which the water flows. They are sometimes attached to the surface of the body, lint are most commonly inclosed within the mantle, or placed in a cavity in its interior called the branehial or respiratory chamber. In many of the bivalves, the openings for the ingress and egress of water are prolonged into tubes or syphons, which arc sometimes of considerable length; the tube through which the water enters being termed the oral syphon, while that through which it escapes is termed the anal syphon. In all the aquatic mollnsca except the cephalop oda, the renewal of the water in contact with the surface of the gills is mainly due to ciliary action. In the air-breathing gasteropodous mollusca (of which the snails and slugs are well-known examples), there is a pulmonary sac or bag into which the air penetrates by an opening on the right side of the body near the neck.
There are considerable differences in the modes of propagation of the mollusca. In the molluscoids—the polyzoa and tutheata—there is both propagation by gemination (like that of zoophytes, q.v.) and sexual reproduction, the sexes being distinct in the polyzoa, and united in the same individual (constituting hermaphroditism. q.v.) in the tunicata. In the lamellibranchiata, or bivalve mollusca, and in the cephalopoda, the sexes are separate; while in the gasteropoda the sexes are most commonly separate, although a considerable number are hermaphrodites, which, however, require mutual impregnation to fertilize the ova. The eggs vary greatly in form; in some cases they are laid separately, but most commonly they are agglutinated together in a mass, while in some marine species many eggs are inclosed in a leathery capsule, while numerous capsules are united to form a large mass. A comparatively few mollusca produce living offspring, the ova being retained in the oviduct until the extrusion of the young animals.
The mollusca are widely diffused through time and space. They were amongst the earliest. animal inhabitants of our globe, and are everywhere found in fresh and salt water (except at great depths), and in every latitude of the earth. The great majority are marine animals, and it is in the tropical regions that the largest and most beautiful forms are developed. It is impossible to form even an approximate estimate of the number of mollusca. According to Leunis (Synopsis der drei Naturreiche ; enter Theft, 1860, p. 77), there are 16,732 living, and 4,590 fossil species, exclusive of polyzoa; and it is probable that only a small proportion of the naked or shell-less mollusca is yet known.
The uses of many species of mollusea for food are too well known to require notice., and as bait for fishing mussels and some other mollusea are of great value.
The animals of thiA sub-kingdom are divisible into the molluseoids and the, true mol lusca, the former being distinguished from the latter by the very low development of the nervous system, which is composed of only a single ganglion, giving off nerves in different directions, and by their propagating by gemmation. The moliuscoids are divisible into: Class 1. POLYZOA or BuvozoA. Examples—Plamatella,' Rostra. Class