366

FORM AND GROWTH OF SHELL.

round a central line in a corkscrew-like mode, a shell is formed like that of the common Snail, or the Pleurotoma (Fig. 582). From forms of this kind, we may return to the Dentalium,

FIG. 558.-SECTION OF

COLUMNARIS.

ACHATINA

which has a long straight cone, like that of the Limpet narrowed and drawn out, by the Scalaria or Wentletrap, in which the coils of the spire touch each other only by their ribs; and by the Magilus and Vermetus (Fig. 583), in which the commencement only of the shell possesses a spiral form, the remainder being prolonged into a tube which nearly approaches the straight direction. When the whorls revolve around a vertical line, instead of remaining in the same plane, a sort of central pillar is formed, which is termed the columella, (Fig. 558); this is usually grooved at its lower part, for the passage of water to the respiratory organs.

909. The margin of the shell is not unfrequently fringed with spines, as in the Murex; these are formed (as are similar ap

pendages in the Conchifera,) by prolongations of the Mantle; and the dissimilar number of them in different specimens has caused the establishment of many species, which, now that the habits of the animal are better known, prove to be but varying forms of the same. For it has been ascertained that the animal has not only the power of forming new spines, but of removing

CHANGES OF SHELL WITH AGE.

367

old ones, especially such as would interfere with the continued growth of the shell. The edge of the Mantle is applied against their bases, and a kind of absorption of shelly matter seems to take place, a notch being formed, which causes them to be easily broken off. Various analogous changes are produced by a similar action in other shells, the portions first formed being wholly or partially removed. Sometimes the walls of the older portions are thinned for the purpose of lightening the shell; and in other cases the top of the cone is altogether removed, a groove having been formed around its interior, which renders it so weak as to be easily broken off; in these last cases, the animal previously withdraws itself from the part that is thus to be separated, and throws a new partition across, by which the top of the shell remains closed after the division. A shell thus deprived of its apex is said to be decollated.

910. It is not only by such removals, that the form of univalve shells undergoes a great change. Sometimes additions are

FIG. 560.-PTEROCERAS SCORPIO: a and b, back and front views of the young shell; e and d, the same of the adult.

made to them, which completely alter their figure, so that two individuals of different ages would be scarcely supposed at first

368

CHANGES OF SHELL WITH AGE.

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 accompanying 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 an 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

FIG. 561.-CYPRAEA EXANTHEMA: a, young shell; b and c, back and front views

of adult shell.

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, incloses 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

SUBDIVISIONS OF GASTEROPODA.

369 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.

911. 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 :

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

II. 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 branchia, which present various forms, on some part of the back.

III. INFEROBRANCHIATA. These are similar in many respects to the preceding, but the branchiæ are situated under the margins of the mantle.

IV. TECTIBRANCHIATA.

In the greater part of the animals contained in this Order, the branchiæ 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.

V. 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 the 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.

VI. 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 inclosed by) other bodies, and is prolonged in the shape of a tube more or less regular.

VII. 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.

VIII. CYCLOBRANCHIATA. These Mollusks 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.

912. Although the greater part of the Mollusks of this Order live on land, some are aquatic; but these, like the aquatic

FIG. 562.-LIMAX RUFUS.

air-breathing Insects and Vertebrata, are obliged to come occasionally to the surface to breathe. They all feed upon vegetables, and many of them do so exclusively; 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 LIMACINE. 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 disk, formed by the mantle, which sometimes incloses a small shell. This shield covers the pulmonary sac, the opening of which is on its right side, and the head can be withdrawn