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4. TECTIBRANCHIATA. In the greater part of the animals contained in this order, the branchise are situated upon the back or on the side, and are covered in by a fold of the mantle, and this fold usually includes a shell more or less developed.

5. HETEROPODA. This is a small order, characterised by the peculiar form of the foot, which is not spread out horizontally, but compressed vertically into a sort of fin. They carry their branchia, the tufts of which are generally protected by a shell, upon the back.

the top of the shell remains closed after the division. | respects to the preceding, but the branchia are situated A shell thus deprived of its apex is said to be decollated. under the margins of the mantle. It is not only by such removals that the form of univalve shells undergoes a great change. Sometimes additions are made to them, which completely alter their figure, so that two individuals of different ages would be scarcely supposed at first sight to belong to the same tribe. But in all these cases the form of the young shell may be traced in that of the adult. The preceding figures of the Pteroceras show this change in a moderate degree; in other genera it is much more remarkable. In another group of shells, of which the common Cowry is an example, a still more curious alteration takes place. In the young shell the edge is sharp, and the mouth or opening of considerable breadth. This state continues as long as the shell is increasing in size; but when it has arrived at adult age, the outer lip is thickened, and brought so near the other as to leave but a narrow chink between them. At the same time, a prolongation of the mantle on each side deposits a new layer of shelly matter on the outside of the previous one; and as the two prolongations meet along the back (the line of their junction being usually evident on the shell), this additional coat, which is very hard and porcellanous in its texture, encloses the whole previous shell.

The operculum is principally confined to the aquatic Gasteropoda. It is sometimes of the same texture as the shell itself, and sometimes horny. It does not always close the entire mouth of the shell, but it is sometimes made to fit it, at all stages of growth, with the most beautiful accuracy. Some of the land species also possess an operculum; but in general they are destitute of it, and they form during hybernation a temporary closure to the mouth of the shell, by a viscid secretion, which hardens into a thin plate, and includes within it a bubble of air. Behind this, a second and even a third similar partition is occasionally found, as in the common snail.

The subdivision of this extensive class into orders, may be best effected by arranging the different tribes according to the character and position of the respiratory organs. The following are those adopted by Cuvier:

1. PULMONEA. These are terrestrial species, adapted to breathe the air by means of a pulmonary sac or aircavity, the orifice of which they can open or close at will. Many have no shell.

2. NUDIBRANCHIATA. These, as well as all the succeeding orders, are aquatic, being adapted to respire water by gills, like other Mollusca. The animals of this order have no shell, and they carry their branchiæ, which present various forms, on some part of the back. 3. INFEROBRANCHIATA. These are similar in many

6. PECTINIBRANCHIATA. The animals of this order, to which belong all the spiral shells, except those of the Pulmonea, are so named from the comb-like form of their gills, which are usually situated in a cavity behind the head, corresponding with the respiratory sac of the Pulmonea. This is by far the most numerous order of the whole.

7. TUBULIBRANCHIATA. These have many affinities with the last order, but the shell is spiral only at its apex, where it is commonly fixed to (or rather enclosed by) other bodies, and is prolonged in the shape of a tube more or less regular.

8. SCUTIBRANCHIATA. In these, also, there is a considerable resemblance to the Pectinibranchiata in the form and position of the gills; but the shells are very open, scarcely in any degree spiral, and cover the body and gills like a shield; and they also differ essentially in their mode of reproduction.

9. CYCLOBRANCHIATA. These molluscs have their gills disposed in little tufts under the margins of the mantle, much as in the Inferobranchiata; but they have shells, which are spread out over the body, and differ from that order in their mode of reproduction.

Order I.-Pulmonea.

Although the greater part of the Molluscs of this order live on land, some are aquatic; but these, like the aquatic air-breathing Insects and Vertebrata, are obliged to come occasionally to the surface to breathe. They all feed chiefly upon vegetables, and many of them exclusively so; but some are extremely voracious, and will devour almost any organised matter that falls in their way. They are diffused through all climates, particular species being restricted to each. Those without a shell, commonly known as Slugs, constitute the family LIMA CINE. In the common Slugs, as in most of the terrestrial species of this order, we observe a prominent head, with four tentacula, and at the end of the longer pair the eyes are situated. These tentacula can be drawn inwards, by a process resembling the inversion of the finger of a glove. On the back there is a kind of shield or disc formed by the mantle, which sometimes encloses a small shell. This shield covers the pulmonary sac, the opening of which is on its right side, and the head can be withdrawn beneath it. The Testacella is a kind of slug which has the disc of the mantle at the posterior extremity, and this always contains a small shell. This animal, which feeds largely on earth-worms, is abundant in the south of France, and has lately been introduced into the gardens of this country, where it is multiplying rapidly.

The Snails and their allies, constituting the family HELICINE, are closely allied to the Slugs in organisation; differing in but little else than the possession of a shell into which the body may be withdrawn. The common garden-snail of this country, and the Helix pomatia, or edible snail of France and Italy, are well-known examples of this family. More striking ones are to be found, however, in tropical climates; where some species of the genus Bulimus attain to great size, the eggs being as large as a pigeon's. In some species the direction of the coils of the shell is opposite to what it is in other spiral shells; such are said to be reversed. A European species of this genus is one most remarkable for the decollation of its shell. Another large snail of tropical climates, as the Achatina, which feeds on trees and shrubs, chiefly on the western coast of Africa, and in

the West Indies. Several of its species are distinguished by the beauty of their colours.

The aquatic Pulmonea have only two tentacula. From the necessity of coming to the surface to breathe, they can only live in water of inconsiderable depth; and they chiefly inhabit ponds and shallow streams, or the banks of rivers. Some, however, live on the seashore. The Planorbis, the shell of which is quite flat, having all its coils upon the same level, is a very common genus in this country; as is also the Lymnæa, which feeds upon seeds, as well as the softer parts of plants, and the stomach of which has a very muscular gizzard.

Order II.-Nudibranchiata.

The animals of this order, which might be designated Sea-Slugs, are all marine; and being adapted to breathe water at any depths, and also, in many instances, to swim with facility, they are often found at a great distance from land. When they swim, it is usually in a reversed position, the foot being turned upwards; this is made concave by muscular action, so as to serve as a kind of boat, the buoyancy of which keeps the animal at the surface without effort. This order is a very numerous one, and some of its species attain a considerable size; but, from their habits, little is known of them.

Order III.-Inferobranchiata.

The few Molluscs contained in this order differ but little from the last, except in the position of their gills, and their incapability of swimming. They are, therefore, confined to the sea-shore; where they subsist, as do the Nudibranchiata, upon sea-weeds and other aquatic plants.

Order IV.-Tectibranchiata.

This order begins to show an approximation towards that disposition of the gills which characterises the great bulk of the class; the animals composing it are marine, and live chiefly on the shore or on floating seaweeds. A very characteristic example of the order is the Aplysia, commonly termed Sea-Hare, which is abundant on many parts of the British coasts. Its vernacular name is probably derived from the peculiar form of the superior pair of tentacula, which are flattened and hollowed, like the ears of a quadruped. The head has a very distinct neck. The branchiae consist of leaflets arranged in a complex form, and situated on the back, beneath a fold of the mantle, which also encloses a flat horny shell. The digestive apparatus is very complicated; consisting of a membranous crop like that of birds, a gizzard having cartilaginous walls, and a third stomach beset with sharp hooks in its interior. These animals feed on sea-weed. They are very sluggish in their movements; but have a peculiar means of defence, consisting of a deep purple liquid, which they can discharge from the edge of the mantle when alarmed, and by which the surrounding water is discoloured, so that they cannot be discerned.

Order V.-Heteropoda.

of the Carinaria inhabits the Mediterranean, and oc sionally appears on particular coasts in large numbʊwhilst others are peculiar to the tropics, where most the allied genera also exist. The shell of the Carina is interesting, on account both of its extreme delen." and fragility, and on account of the strong resemb which that of the Argonaut or Paper-Nautilus bears

Order VI.-Pectinibranchiata

This order is not only by far the most numerous the whole class, but contains the animals which be regarded as its most characteristic examples. 1 have all two tentacula and two eyes, sometimes ra.on stalks, as in the snail. The mouth is prolongre a sort of proboscis; and the tongue is furnished little hooks or recurved spines, which enable it to down the hardest bodies by slow and oft-repeate tion. The cavity in which the gills are fixed oer.” the last whorl of the shell; and in some of the there is a tubular prolongation of the mantle, ter the siphon, for the purpose of conveying water int cavity, so that the animal can breathe without lea its shelter. By the presence or absence of thus and by the form of the shell, which here appear bear a sufficiently constant relation with that animals, this large group may be arranged under following families:-1. TROCHOIDE, in which there » siphon, and which have the mouth closed by an e culum. Of this the common periwinkle is a char. ristic example, though very small in proportion t pical species. 2. CAPULOIDE, which have a wide shell, very much like that of the limpets, without culum or notch at the margin for the passage siphon. 3. BUCCINOIDE, which have a spiral she & a canal at the end of the columella for the pass the siphon; this is sometimes extremely pro.cfg in the Murex; and the genera exhibiting this chara are all carnivorous in their habits. To tas ta belong the animals forming the greatest numb marine univalve shells preserved in cabinets.

Order VIL-Tubulibranchiata.

The Molluscs of this order construct an irrega tubular shell, which so much resembles that f by certain Annelida, as to be scarcely distinguis from it. They are very few in number.

Order VIII.-Scutibranchiata.

This order is also a small one, containing but principal genera, which do not differ widely free limpets, except in the disposition of the gills. Thes are very open, without an operculum, and the gre number are not in any degree spiral. In the Hay the shell is slightly twisted; and from a faint re blance it is thought to bear to the ear of a quadr it has been called the Sea-Ear. This anima living state, is one of the most beautiful of Gaster on account of the variety and richness of its e Its shell, when the surface is polished, possess pearly lustre, with resplendent metallic hues. 1. consequently much sought for as an ornament.

Order IX.-Cyclobranchiata.

This is a very small order; but the animals contained in it differ remarkably from all other molluscs. At the edge of the vertical muscular plate, which has been The general form of the Limpets, which princi mentioned as occupying the position of the foot in the compose this order, is well known; and the pecua other Gasteropoda, is a small conical sucker, by which in the position of their gills has already been menta the animal can attach itself, and which represents the Closely allied to the limpets in general structure, expanded disc of other orders. The gills are placed differing remarkably in the formation of the shelon the back, and close behind them are the heart and the Chitons, of which some small species inhab* liver, which seem, as it were, external to the body. shores, but which attain to much greater size be In one of the few genera which the order contains, the tropics. Their shell is composed of a numb" these parts are protected by a small shell, whilst in plates arranged behind one another with great repanother there is none. The body, consisting of a semi-rity, and connected by a very complex series of transparent gelatinous substance, enclosed in a muscular envelope, is elongated, and generally terminated by pressed tail; this body can be distended with e animal swims in a reversed position, the fining uppermost, and the shell depending bey are limited to warm latitudes: one species

ments and muscles, which reminds the natura those which unite and move the different segments the articulated animals.

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ZOOLOGY-(CONCLUDED).

ligament, which is composed of an elastic animal sub- Interior of Mussel: a, right valve; b, left valve; c, hinge; d, sto

Order V.-Inclusa.

and which, therefore, they cannot quit. By what means they accomplish this is unknown. The flattened form of the shell is a sufficient proof that it is not by mecha- In the order Inclusa we usually find the valves, nical action, as in some of the boring shells of the order when joined together, presenting more or less of a Inclusa. The Anodon is a fresh-water mussel, closely cylindrical form, as in the common Razor-shell. A allied in general conformation to those inhabiting the hollow is left at each end, and from one the foot is prosea, but differing remarkably in the absence of teeth injected, through a passage in the mantle, whilst the other the hinge, whence the name of the genus is derived. gives exit to the respiratory tubes, which are often The Unio is another fresh-water genus, having a more prolonged to a great length. The animals live almost complicated hinge. It is remarkable for the pearly uniformly buried in sand or mud, in rocks or wood. aspect of the lining of the valves, and for producing The Solen, or Razor-shell, is a very characteristic exsmall pearls, sometimes in considerable abundance. ample of the order, which contains, however, some These are not so pure in their colour, however, as those forms that depart very widely from it. The foot, which of the Avicula, and are but little esteemed. Some spe- | can be projected from the lower end, is firm and pointed, cies of Unio are common in the lakes and rivers of and serves as an admirable boring instrument, by the Britain, but the greater number are peculiar to North use of which the animal can burrow in the sand with America; many of these are remarkable for their size great rapidity, sinking very deep when alarmed. The and colour. There are some marine Mytilaces allied Mya approaches, in the form of its shell, and in its to the Unios, one of which burrows in coral. general organisation, to the previous family; but some of its species also closely approximate the Solens. The Pholas is a very interesting genus, the animal of which nearly resembles that of the Solen, whilst the shell is formed of several pieces, and would thence be called a multivalve. There are two principal portions, and a variable number of accessory pieces. Some species of this genus bore in mud, others in rocks, and a few in wood. Their action seems purely mechanical. They fix themselves firmly by the powerful foot, and then make the shell revolve; the sharp edges of this commence the perforation, which is afterwards enlarged by the rasp-like action of the rough exterior; and though the shell must thus be constantly worn down, yet it is replaced by a new formation from the animal, so as never to be unfit for its purpose.

Order III.-Camaccæ.

The order Camaceæ includes but a comparatively small number of species, most of which are peculiar to tropical climates. The most remarkable is the Tridacne | gigas, or Giant Clamp-shell, of the Indian Ocean. When young, this animal attaches itself to rocks by means of its tendinous foot, which serves as a sort of byssus; but when its shell becomes so massive that it is in no danger of injury, it detaches itself, and the groove of the shell is filled up. These animals, with their shells, sometimes attain the weight of many hundred pounds. Smaller specimens are often brought to this country; the valves being used to receive water from small fountains, &c.; and, on the continent, they are employed as reservoirs of holy water in the churches. The foot has a structure Bo tough, that, to separate the shell attached by it, it is necessary to chop it with a hatchet like a cable.

Order IV.-Cardiaceæ.

We now pass on to some very remarkable forms of this order, in which the bivalve character of the shell gradually disappears, being replaced by a new structure of which no examples have been yet seen. The Teredo, or Wood-worm, as it is commonly termed, is an animal of the same general organisation as the Pholas; but its In the animals of the order CARDIACEE, or Cockle valves are smaller in proportion to the body, and its tribe, we usually find greater activity than in any others tubes still more prolonged-at least when the animal of the class. The foot now comes to be a very impor- may be regarded as full-grown. By means of the tant organ, possessed of great muscular power, and mechanical action of its valves, it perforates timber, in capable of being applied to a variety of uses. Many of the same manner as the Pholas penetrates stone. As these animals bury themselves in sand or mud; and it it advances, the respiratory tubes are prolonged, so that is in these that we find the respiratory orifices prolonged their orifice remains at the entrance of the burrow, into tubes. In the common Cockle, however, these which is very small in proportion to the cavity formed tubes can scarcely be said to exist, the orifices not being by the animal as it increases in size. This orifice is prolonged beyond the shell. Its foot is very large, and furnished with a pair of valve-like shelly plates, termed can be bent nearly double in the middle; by doing this, palmule; by the action of which a current of water is and then suddenly straightening it, the animal is enabled driven towards the body of the animal, in order to serve to take considerable leaps. The hinge is very beauti- for its respiration, and at the same time for the supply fully constructed; and the two valves lock closely to- of its food. The gallery is lined by a calcareous exugether. The Trigonia, a very interesting genus, abun-dation from the surface of the tubular prolongation of dant in former ages of the globe, but now restricted to the shores of New Holland, seems allied to the cockle in the structure of its hinge and shell, in the size of its foot, and in its general organisation, although its mantle is divided into two lobes along the whole margin of the valves, as in the oyster. This is an instance of the impropriety of basing our classification on any single characters. There are other Cardiaceae which obviously conduct towards the next order. Such is the Tellina, of which the valves, when closed together, gape at the posterior extremity; and through this passage there pass out two long tubes which terminate in the branchial orifices and funnel. These tubes have an elastic structure, and can be entirely drawn within the shell. These long tubes are seen also in the Venus and its allies, of which one species is remarkable for the long spines which guard its posterior end; and also in the Mactra, and the animals allied to it. In some cases the tubes are united along their whole length, and appear to be single; but they are always in reality double. All these animals are in the habit of burying themselves in sand or mud, or in stones; and the tubes serve to introduce fresh water from the entrance of their bur

rows.

the mantle, which forms a kind of secondary shell. The Teredo is an animal extremely destructive to timber, especially in warm climates, from which it seems to have been originally introduced into the scaports of Europe. In other genera the valves are lost in the shelly tube, so that the ordinary structure of the class is no longer apparent.

Besides the orders now described as composing the class Conchifera, there is a very curious group which should also be included in it, although established as a separate class by many naturalists. This is the group of BRACHIOPODA, containing only three genera at present known, namely, Terebratula, Lingula, and Orbicula; but formerly of much greater comparative importance. These animals have all bivalve shells, differing in no essential particular from those of the Conchifera in general. The two former genera are attached, however, by a footstalk proceeding from an opening in one of the valves near the hinge, to solid substances; and in this respect they have an affinity with the Tunicata. The Orbicula is attached, like an oyster, by one of its valves. They bear considerable resemblance to the Tunicata, also, in the structure of the nutritive system-the digestive apparatus, heart, gills, &c. But