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is the one which remains, and the anterior adductor is absent. The adductors leave distinct "muscular impressions" in the interior of the shell, so that it is easy to determine whether there has been one only in any given specimen, or whether two were present.

The habits of the Lamellibranchiata are very various. Some, such as the Oyster (Ostrea), and the Scallop (Pecten), habitually lie on one side, the lower valve being the deepest, and the foot being wanting, or rudimentary. The former is fixed by the substance of the valve, but the latter swims by rapidly opening and closing the shell. Others, such as the Mussel (Mytilus) and the Pinna, are attached to some foreign object by an apparatus of threads, which is called the "byssus," and is secreted by a special gland. Others are fixed to some solid body by the substance of one of the valves. Many, such as the Myas, spend their existence sunk in the sand of the seashore or in the mud of estuaries. Others, as the Pholades and Lithodomi, bore holes in rock or wood, in which they live. Finally, many are permanently free and locomotive.

The Lamellibranchiata may be divided into two sections, according as respiratory siphons are absent or present, as follows:

SECTION A. ASIPHONIDA. Animal without respiratory siphons; mantle - lobes free; the pallial line simple and not indented (Integropallialia).

This section comprises the families Ostreida, Aviculida, Mytilida, Arcade, Trigoniada, and Unionida.

The Ostreida (including the Oysters, Scallops, Anomia, Thorny Oysters, &c.) are all marine, and are monomyary. The Aviculida, or Pearl-Oysters, are likewise marine, but are dimyary. The Mytilida (Mussels, HorseMussels, &c.) are partially marine and partially fresh-water forms, and have a very small anterior adductor. The Arcade (Ark-shells, &c.) are exclusively marine, as are the nearly allied Trigoniada. Lastly, the Unionida (Fresh-water Mussels) are exclusively confined to rivers and lakes.

SECTION B. SIPHONIDA.—Animal with respiratory siphons ; mantle-lobes more or less united.

Two subdivisions, of little classificatory or anatomical value, are comprised in this section. In the first the siphons are short, and the pallial line is simple (Integropallialia); as is seen in the families Chamide, Hippuritida, Tridacnida, Cardiada, Lucinide, Cycladida, and Cyprinidæ.

The second subdivision (Sinupallialia) is distinguished by the possession of long respiratory siphons, and a sinuated pallial line, and it comprises the families Venerida, Mactrida, Tellinida, Solenidae, Myacida, Anatinida, Gastrochanida, and Pholadida.

The Chamida (Thorny Clams) are fixed to foreign bodies by the substance of either valve indifferently, and are all inhabitants of the sea. The extraordinary extinct group of the Hippuritide, from the fossils associated with them, are known to have been also marine; and they are often found in great beds like Oysters, attached to one another and to foreign objects by the beak of the right valve. The Tridacnide (Giant Clams) have a similar habitat, and the shell may attain a weight of five hundred pounds. The Cardiada (Cockles) and Lucinide are also marine, as are the Cyprinida; but the Cycladida are fresh-water and brackish-water forms. The Venerida (Clams) are amongst the most beautiful of the Bivalves, and are found in all seas, attaining their maximum in warm regions. The Mactrida (Trough-shells) and Tellinida are mostly marine, though also found in brackish waters; and the Solenida (Razor-shells), Myacida, and Anatinidæ are essentially marine, though some of Myacida extend their range for a considerable distance above the mouths of rivers. The Gastrochenida are all natives of the sea, and have a burrowing habit, boring holes for habitation in rocks, or living in the mud. Lastly, the Phol adida (Piddocks and Ship-worms) bore holes in stone or wood, in which they live, and are all marine in habit. The Ship-worms (Teredo) have long worm-like bodies, and do an immense amount of harm by honey. combing with their burrows the sides of ships, or other wooden structures immersed in the sea.

DISTRIBUTION OF THE LAMELLI BRANCHIATA IN TIME.-The Lamellibranchs are known to have existed in the Upper Cambrian period, and have steadily increased up to the present day, when the class appears to have attained its maximum, both as regards numbers and as regards variety of type. The recent bivalves are also superior in organisation to those which have preceded them. Upon the whole, the Asiphonate bivalves are more characteristically Paleozoic, whilst those in which the mantle-lobes are united, and there are respiratory siphons, are chiefly found in the Secondary and Tertiary epochs. One very singular and aberrant family-viz., the Hippuritide-is exclusively confined to the Secondary rocks, and is, indeed, not known to occur beyond the limits of the Cretaceous formation. The Venerida, which are perhaps the most highly organised of the families of the Lamellibranchiata, appear for the first time in the Oolitic rocks, and, increasing in the Tertiary period, have culminated in the recent period.

CHAPTER XLVI.

GASTEROPODA.

DIVISION ENCEPHALA, or CEPHALOPHORA.—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 Pteropoda, 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 effected by means of a broad, horizontally flattened, ventral disc-the "foot; " or by a vertically flattened, ventral, fin-like organ. Flexure of intestine hamal or neural.

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This class includes all those Molluscous animals which have a shell of a single piece, and are commonly known as valves," such as the land-snails, sea-snails, whelks, limpets, &c. 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 Lamellibranchiata, 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 (Strombida), 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.

In some, again, the upper and lateral surfaces of the foot

are expanded into muscular side-lobes, which are called "epipodia." In many cases the metapodium, or posterior portion

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Fig. 194.-A, Sketch of a Whelk (Buccinum undatum) in motion: ƒ Foot; & Head carrying the feelers (t) with the eyes (e) at their bases; Proboscis; s Respiratory siphon, or tube by which water is admitted to the gills; o Operculum. B, Shell of the Whelk; a Spire; b Body-whorl ; # Notch in the front margin of the mouth of the shell; m Outer lip of the mouth of the shell. This figure is half the natural size. C, A small cluster of the egg-capsules of the Whelk. (B and C are after Woodward.)

of the foot, secretes a calcareous, horny, or fibrous plate, which is called the "operculum" (fig. 197, 0), and which serves to close the orifice of the shell when the animal is retracted within it.

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) always provided with a singular masticatory apparatus, which is var ously known as the "lingual ribbon," the "tongue," the "odontophore," or the "radula." This consists of a longer o shorter ribbon-shaped structure, which is attached behind t the bottom of a secreting sac or sheath, situated on the lowe

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. 195). 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 (fig. 196), 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 "hæmal," 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.

Fig.

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

A distinct heart (fig. 196, h) 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 lacunæ 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 (Branchiogasteropoda) 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 plume-like branchiæ, 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

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