The Mollusca may be defined as including soft-bodied, unsegmented animals, which are usually provided with an exoskeleton. The intestinal canal is bounded by its own proper walls, and is completely shut off from the perivisceral cavity. The alimentary canal is situated between the haemal system, which lies dorsally, and the neural system, which is situated towards the ventral aspect of the body. The nervous system (fig. 196) in its highest development consists of three principal ganglia, which are reduced to one in the lower forms. Usually there is a distinct propulsive organ by which the circulation is carried on, but this is occasionally absent. Distinct respiratory organs may or may not be present. Reproduction is sexual, though gemmation is also occasionally superadded. The higher Mollusca are all simple animals, but many of the lower forms are capable of forming colonies by continuous gemmation.
The digestive system in all the Mollusca consists of a mouth, gullet, stomach, intestine, and anus - though in some of the Brachiopoda, and in a few other forms, the intestine ends caecally. In some the mouth is surrounded by ciliated tentacles (Polyzoa, fig. 199); in others it is furnished with two ciliated arms (Brachiopoda, fig. 204); in the bivalves (Lamelli-branchiata) it is mostly furnished with four membranous processes or palpi (fig. 208)', in others it is provided with a cornplicated apparatus of teeth (Gasteropoda, fig. 213, and Ptero-poda); and, lastly, the Cephalopoda have, in addition, horny or calcareous mandibles, forming a kind of beak. Well-developed salivary glands are usually present; the liver in the higher forms is of large size, and pours its secretion either into the stomach or into the commencement of the intestine; and a renal organ has been detected in most of the Mollusca proper. There is no distinct absorbent system, but the products of digestion pass by exosmose into the general abdominal cavity, and thence into the larger veins.
The blood is colourless, or nearly so. In the Polyzoa the circulation is carried on by ciliary action, and there is no distinct propulsive organ, or definite course of the circulating fluid. In the Tunicata the heart is a simple tube, open at both ends, and the course of the circulation is periodically reversed. In the Brachiopoda the course of the circulation is not definitely ascertained, and it is doubtful if a true heart is present in all. In the higher Mollusca a distinct heart is always present, and consists of an auricle which receives the aerated blood from the breathing-organ, and a muscular ventricle which propels it through the systemic vessels. That a system of capillaries in many cases intervenes between the arteries and veins, appears from recent researches to be probable. In all cases the heart of the Mollusca is systemic, distributing the aerated blood to the body; and in no case is it respiratory, propelling the non-aerated blood to the breathing-organ.
In the Polyzoa there is no differentiated respiratory organ, and the function of respiration is discharged mainly by the oral crown of ciliated tentacles. In the Tunicata respiration is effected by means of the pharyngeal or branchial sac; and in the Brachiopoda by the oral arms, and possibly, to some extent, by an "atrial" or "water-vascular" system, furnished with contractile dilatations. In the higher Mollusca a distinct breathing-organ is always present, a portion of the mantle being specialised for this purpose. In the Lamellibranchiata, and the branchiate Gasteropoda, the breathing-organs are in the form of lamellar and pectinate gills; and the same is the case with the Cephalopoda. In the pulmonate Gasteropoda, in which respiration is aerial, a pulmonary sac or air-chamber is produced by the folding of a portion of the mantle, over the interior of which the pulmonary vessels are distributed. The chamber thus formed communicates with the exterior by a round aperture, which can be opened or closed at will; and the renovation of the effete air within the sac appears to be effected mainly or entirely by simple diffusion.
The nervous system varies considerably in its development. In the Polyzoa, Tunicata, and Brachiopoda - which collectively constitute the Mollnscoida - the nervous system consists of a single ganglion, or of a principal pair with accessory ganglia, placed between the oral and anal apertures. The true Molluscan type (fig. 196), however, of nervous system is constituted by the presence of three pairs of ganglia, connected with one another by commissures, but distributed in a characteristically scattered manner (hetero-gangliate type). One of these ganglia is situated above the oesophagus, and is called the "supra-oesophageal" or "cerebral" ganglion. A second is placed below the oesophagus, and is termed the "infra-oesophageal" or "pedal" ganglion (from its supplying the nerves to the " foot"). The third pair is the most persistent, and is termed the "branchial" or "parieto-splanchnic " ganglion. In many of the higher Mollusca, however, the cerebral and pedal ganglia are fused in an oesophageal ring.
Organs of sight exist in some of the lower, and in the majority of the higher, Mollusca. In the Cephalopoda, and in some of the Gasteropoda (e.g., Strombidae), the eyes are of a very high type of organisation. In the Lamellibranchiata the adults are either destitute of organs of vision, or possess numerous simple eyes ("ocelli") placed along the margins of the mantle-lobes. Similar ocelli are also found in some of the Tunicata, placed between the oral tentacles. Organs of hearing ("otocysts") exist in the more highly organised Mollusca, especially in the Gasteropoda and Cephalopoda, and supposed olfactory organs occur in some of the latter.
Reproduction amongst the Mollusca is almost invariably sexual, but it is by continuous gemmation that the colonies of the Polyzoa, and the social and compound Tunicata, are produced, and the "statoblasts" of the former offer a good example of non-sexual reproduction. The sexes may be distinct, or are in many cases united in the same individual. In many forms the ova are arranged in rows, so as to form a strap or ribbon - shaped structure, termed the "nidamental ribbon." There is generally a distinct metamorphosis in development.
As implied by their scientific name, the Mollusca are mostly soft-bodied animals; but their popular name of " Shell-fish " expresses the fact, that the presence of a shell, protecting the soft body, is likewise a very characteristic feature in the sub-kingdom. At the same time, a shell is not universally present, and many of the Mollusca are either permanently naked, or possess nothing that would be ordinarily looked upon as a shell. When there is either no shell at all, or merely a rudimentary shell enclosed in the mantle, the Mollusc is said to be "naked." The shell of the "testaceous" Mollusca is very closely related to the respiratory organs; "indeed it may be regarded as a pneumoskeleton. being essentially a calcified portion of the mantle, of which the breathing-organ is at most a specialised part. . . . In its most reduced form the shell is only a hollow cone or plate, protecting the breathing-organ and heart, as in Limax, Testacella, and Carinaria. Its peculiar features always relate to the condition of the breathing-organ, and in Terebratula and Pelonaia it becomes identified with the gill. In the Nudibranchs the vascular mantle performs, wholly or in part, the respiratory office. In the Cephalopods the shell becomes complicated by the addition of a distinct, internal, chambered portion (phragmacone), which is properly a visceral skeleton" (Woodward). In a great many of the Mollusca proper the shell consists of but a single piece, and they are called "univalves." In many others the shell consists of two separate plates or "valves," and these are called "bivalves." In others, again, as in the Chiton, the shell consists of more than two pieces, and is said to be "multivalve." Most, however, of the multivalve shells of older writers are in reality referable to the Cirripedia.
All the testaceous Mollusca (except the Argonaut), and most of the "naked " forms, acquire a rudimentary shell before their liberation from the ovum. In the latter this rudimentary shell is cast off as the embryo grows, but in the former it becomes the "nucleus" of the adult shell. In the bivalves the embryonic shell or "nucleus" is situated at the beak or "umbo" of each valve, and is often very unlike the remainder of the shell.
In composition the shell of the Mollusca consists of carbonate of lime - usually having the atomic arrangement of calcite - with a small proportion of animal matter. In the Pholadidae, however, the calcareous matter exists in the allotropic condition of arragonite, which is very much harder than calcite; and there are many Gasteropods in which the shell is similarly composed of arragonite. As regards their texture, three principal varieties of shells may be distinguished - viz., the "porcellanous," the "nacreous," and the "fibrous." In the " nacreous " or pearly shells, as seen in " mother-of-pearl" the shell has a peculiar lustre, due to the minute undulations of the edges of alternate layers of carbonate of lime and membrane. The " fibrous " shells are composed of successive layers of prismatic cells. The "porcellanous" shell has a more complicated structure, and is composed of three layers or strata, each of which is made up of very numerous plates, "like cards placed on edge." The direction in which the vertical plates are placed, is sometimes transverse in the central layer, and lengthwise in the two others; or longitudinal in the middle, and transverse in the outer and inner strata.
All living shells have an outer layer of animal matter, which is known as the "epidermis," or "periostracum." This is sometimes of extreme tenuity, but is sometimes very thick, the latter being especially the case with those shells which are found in fresh water.
In many of the spiral univalves, as the animal grows it withdraws itself from the upper portion of the shell, often partitioning off the space thus left vacant. In many instances the portion thus abandoned falls off, and the shell becomes "truncated," or " decollated ; " this being the normal condition in fully-grown examples of some shells.
In the great majority of univalves the shell is coiled into a spiral, the direction of which is right-handed, but in some cases the spiral is left-handed, and the shell is said to be "reversed," or "sinistral." There-versed shell may occur as the normal condition of the species, or it may occur simply as a variety of a form which is normally right-handed, or "dextral."
The sub-kingdom Mollusca is divided into two great divisions, termed respectively the Molluscoida, and the Mollusca proper. In the former of these the nervous system consists of a single ganglion or principal pair of ganglia, and there is either no circulatory organ or an imperfect heart. In the latter the nervous system consists of three principal pairs of ganglia, and there is a well-developed heart, consisting of at least two chambers.