The remaining three classes of the Mollusca proper all possess a distinctly differentiated head, and all are provided with a peculiar masticatory apparatus, which is known as the "odontophore." For the first of these reasons they are often grouped together under the name Encephala; and for the second reason they are united by Huxley into a single great division, under the name of Odontophora. Whichever name be adopted, the three classes in question (viz., the Gasteropoda, Pteropoda, and Cephalopoda) certainly show many points of affinity, and form a very natural division of the Mollusca. The Pieropoda, as being the lowest class, should properly be treated of first, but it will conduce to a clearer understanding of their characters if the Gasteropoda are considered first.

Class II. Gasteropoda

The members of this class are never included in a bivalve shell; locomotion is effected by means of a broad, horizontally flattened, ventral disc - the "foot; " or by a vertically flattened, ventral, fin-like organ. Flexure of intestine haemal or neural.

This class includes all those Molluscous animals which have a shell of a single piece, and are commonly known as " univalves," such as the Land-snails, Sea-snails, Whelks, Limpets, etc. The shell, however, is sometimes composed of several pieces (multivalve), and in many there is either no shell at all, or nothing that would be generally recognised as such. In none is there a bivalve shell. The Gasteropods may be regarded as the most typical of the Mollusca, though not the most highly organised. All of them have a body composed of three principal portions - a head, foot, and visceral sac - the last of these being enclosed in the integumentary expansion known as the "mantle." In all, except the few sedentary forms, the "foot " is the organ of locomotion.

In most of the Gasteropoda the body is unsymmetrical, and is coiled up spirally, "the respiratory organs of the left side being usually atrophied" (Woodward). The body is enclosed in a "mantle," which is not divided into two lobes as in the Lamel-libranchiata, but is continuous round the body. Locomotion is effected by means of the "foot," which is usually a broad muscular disc, developed upon the ventral surface of the body, and not exhibiting any distinct division into parts. In the Heteropoda, however, and in the Wing-shells (Strombidae), the foot exhibits a division into three portions: an anterior, the "propodium;" a middle, the "mesopodium;" and a posterior lobe, or " metapodium." In the Heteropoda, the foot is flattened, and forms a ventral fin, by means of which the animal swims, back downwards.

A distinct heart is almost always present, composed of an auricle and ventricle. In many Gasteropods it has been shown that the blood-vessels form closed tubes, and that the arteries and veins are connected by an intermediate system of capillaries, instead of merely communicating through the interstices and lacunae between the tissues. It seems also certain that, in general at any rate, there is no direct connection between the blood-vessels and the outer medium, though, in some cases, such a communication seems undoubtedly to exist. Respiration is very variously effected; one great division (Branchio-gasteropoda) being constructed to breathe air by means of water; whilst in another section (Pulmogasteropoda) the respiration is aerial. In the former division respiration may be effected in three ways. Firstly, there may be no specialised respiratory organ, the blood being simply exposed to the water in the thin walls of the mantle-cavity (as in some of the Heteropoda). Secondly, the respiratory organs may be in the form of outward processes of the integument, exposed in tufts on the back and sides of the animal (as in the Nudibranchiata). Thirdly, the respiratory organs are in the form of pectinated or plumelike branchiae, contained in a more or less complete branchial chamber formed by an inflection of the mantle. In many members of this last section the water obtains access to the gills by means of a tubular prolongation or folding of the mantle, forming a "siphon" (fig. 214, s), the effete water being expelled by another posterior siphon similarly constructed. In the air-breathing Gasteropods, the breathing organ is in the

The head in most of the Gasteropoda is very distinctly marked out, and is provided with two tentacles and with two eyes, which are often placed upon long stalks. Very often there is an elongated retractile proboscis, with ear-sacs, containing otoliths, at its base. The mouth is sometimes furnished with horny jaws, and is (with extremely few exceptions) provided with a singular masticatory apparatus, which is variously known as the "lingual ribbon," the "tongue," the "odonto-phore," or the "radula." This consists of a longer or shorter ribbon - shaped structure, which is attached behind to the bottom of a secreting sac or sheath, situated on the lower wall of the pharynx posteriorly. The lingual ribbon extends forwards along the inferior wall of the pharynx, being supported by a species of cartilaginous cushion, over which it can be made to work backwards and forwards by appropriate muscles. It carries a great number of hook-shaped teeth arranged in transverse rows, there generally being a principal central and two or more lateral rows (fig. 213). These teeth formerly supposed to be siliceous, are now known to be mainly chitinous, and their form and disposition are so various and so constant in different forms, that they afford very valuable help in classification. The mouth leads by a gullet into a distinct stomach, which is sometimes provided with cartilaginous or calcareous plates for the trituration of the food. The intestine is long, and its first flexure is commonly "haemal," or towards that side of the body on which the heart is situated; though in some the flexure is "neural." Distinct salivary glands are usually present, and the liver is well developed.

A distinct heart is almost always present, composed of an auricle and ventricle. In many Gasteropods it has been shown that the blood-vessels form closed tubes, and that the arteries and veins are connected by an intermediate system of capillaries, instead of merely communicating through the interstices and lacunae between the tissues. It seems also certain that, in general at any rate, there is no direct connection between the blood-vessels and the outer medium, though, in some cases, such a communication seems undoubtedly to exist. Respiration is very variously effected; one great division (Branchio-gasteropoda) being constructed to breathe air by means of water; whilst in another section (Pulmogasteropoda) the respiration is aerial. In the former division respiration may be effected in three ways. Firstly, there may be no specialised respiratory organ, the blood being simply exposed to the water in the thin walls of the mantle-cavity (as in some of the Heteropoda). Secondly, the respiratory organs may be in the form of outward processes of the integument, exposed in tufts on the back and sides of the animal (as in the Nudibranchiata). Thirdly, the respiratory organs are in the form of pectinated or plumelike branchiae, contained in a more or less complete branchial chamber formed by an inflection of the mantle. In many members of this last section the water obtains access to the gills by means of a tubular prolongation or folding of the mantle, forming a "siphon" (fig. 214, s), the effete water being expelled by another posterior siphon similarly constructed. In the air-breathing Gasteropods, the breathing organ is in the form of a pulmonary chamber, formed by an inflection of the mantle, and having a distinct aperture for the admission of air.

Fig. 213.   Fragment of the lingual ribbon or odontophore of the common Whelk (Buccinum undatum), magnified. (After Woodward.)

Fig. 213. - Fragment of the lingual ribbon or odontophore of the common Whelk (Buccinum undatum), magnified. (After Woodward.)

Fig. 214.   Ampullaria canaliculata, one of the Apple shells, o Operculum ; s Respiratory siphon.

Fig. 214. - Ampullaria canaliculata, one of the Apple-shells, o Operculum ; s Respiratory siphon.

The nervous system in the Gasteropoda has its normal composition of three principal pairs of ganglia, the supra-oesopha-geal or cerebral, the infra-oesophageal or pedal, and the parieto-splanchnic; but there is a tendency to the aggregation of these in the neighbourhood of the head. The organs of sense" are the two eyes, and auditory capsules placed at the bases of the tentacles, the latter being tactile organs.

The sexes are mostly distinct, but in some they are united in the same individual. The young, when first hatched, are always provided with an embryonic shell, which in the adult may become concealed in a fold of the mantle, or may be entirely lost. In the branchiate Gasteropods the embryo (fig. 215, A) is protected by a small nautiloid shell, within which it can entirely retract itself; and it is enabled to swim freely by means of a ciliated, often lobed extension of the cephalic integument, which is termed the "velum," and which is at first merely a circlet of cilia round the head. The velum has often been compared with the wing-like cephalic fins of the Ptero-poda, with which, however, it is only doubtfully homologous. Amongst the Pulmonate Gasteropods, those which are strictly terrestrial, undergo no metamorphosis, the velum being absent altogether, whereas those that live in fresh water possess structures which correspond with the velum of the Branchiate forms.