ORDER VI.

TUBULIBRANCHIATA.

926. The Mollusks of this Order construct an irregularly tubular shell; which so much resembles that formed by certain Annelida, as to be scarcely distinguishable from it. They are very few in number. The Vermetus (Fig. 583) is the principal genus of the group. This is remarkable for the close resemblance

FIG. 583.-VERMETUS.

of its shell to that of the Serpula (Fig. 528); but, when perfect, it may be generally distinguished by the regularly-spiral twisting of its first-formed portion. Some species associate together in large masses, so as even to form reefs; whilst others attach themselves to Coral, and lengthen their shells in proportion as the Coral grows up around them. When the animal has quitted the lower part of its tube, that it may keep itself at the surface, it usually throws a partition across, in the manner of the Nautilus; and a tolerably regular series of such partitions is not unfrequently found. In the Magilus, an allied genus of similar habits, whose tube is sometimes lengthened in this manner to as much as three feet, the first-formed part of the shell, instead of being cut off, is sometimes completely filled up by the exudation of solid matter.

ORDER VII.-SCUTIBRANCHIATA.

927. This Order is also a small one, containing but two principal genera, which do not differ widely from the Limpets, except in the disposition of the gills. The shells are very open, without an operculum, and the greater number are not in any degree

HALIOTIS.-CYCLOBRANCHIATA; LIMPETS; CHITONS.

387

spiral. In the Haliotis, the shell is slightly twisted; and from a faint resemblance it is thought to bear to the ear of a quadruped, it has been called the Sea Ear. This animal, in its living state, is one of the most beautiful of Gasteropods, on account of the variety and richness of its colours. Its shell, when the surface is polished, possesses a pearly lustre, with resplendent metallic hues. It is consequently much sought for as an ornament. The animal attaches itself to the rock, after the manner of a Limpet, by a large muscular foot; and the extent of the adhering surface is so great, that a very considerable force is required to detach it, when the animal is putting forth its strength. The best mode of obtaining the shell, is to place the hand or some instrument under its edge (which is usually a little separated from the rock when the animal is undisturbed); and to endeavour to remove it before the animal is alarmed. When this attempt is not successful, the animal draws its shell down upon the rock with such firmness, as to grasp most securely anything that is under its edge; and in this manner fatal accidents have occurred to men, who have incautiously attempted to remove these shells, when under water.

ORDER VIII.-CYCLOBRANCHIATA.

928. The general form of the Limpets, which principally compose this Order, is well known; and the peculiarity in the position of their gills has already been mentioned. Closely allied to the Limpets in general structure, but differing remarkably in the formation of the shell, are the Chitons; of which some small species inhabit our shores, but which attain to much greater size between the tropics. Their shell (Fig. 584) is composed of a number of plates arranged behind one another with great regularity, and connected by a very complex series of ligaments and muscles, which reminds the Naturalist of those which unite and move the different segments in the Articulated animals. The Limpets and Chitons fix themselves to the rock, in the same manner as the Haliotis; and it is stated that Crows and other birds,

388

LIMPETS.-GEOLOGICAL DISTRIBUTION.

which endeavour to detach them for food, are sometimes caught by the points of their bills, and are held there until they are drowned by the advancing tide. The Limpets are herbi

vorous, feeding upon sea-weeds, which they reduce with their long riband-shaped rasp-like tongues. Although we usually see them attached quite motionless to rocks, this is because exposure to the air is unfavourable to their movement, since too free an admission of it between their gills would dry them up. It is when covered with water, that their activity manifests itself. In many points of structure, the animals of this Order approach those of Bivalve shells. The rock, in the Limpet, may almost be regarded as a second valve; since the muscle takes a firm attachment to its surface, and draws down the shell upon it, in the same manner as the adductor muscle of the Bivalves enables the animal to inclose itself by drawing the two valves of the shell together (§ 932). Moreover the eyes are very imperfect or entirely absent; and many other points of internal structure prove the inferiority of these animals to the Gasteropods in general.

FIG. 584.-CHITON.

929. To enter into any detailed account of the Fossil Remains of this class, would be unsuitable to our present purpose; and it will be sufficient here to state some general facts in regard to them. Remains of Univalve shells, of such a form and structure as evidently to have belonged to Gasteropod Mollusks, are found in nearly every bed formed by the action of water, from the very earliest of those containing fossils of any kind, down to the present time. In many instances, the fossil shells, even of the most ancient beds, may be referred to genera which still exist;-thus a species of Buccinum is found in the rocks of the Silurian system;-although they do not correspond with any species now living, except in cases where there is reason to believe that the deposit was formed at a comparatively recent period.

GEOLOGICAL DISTRIBUTION OF GASTEROPODA.

389

The Conchologist is generally able to determine, by an examination of the shell, whether it was formed by a marine or a freshwater Mollusk; and in this manner he often receives important guidance, in determining the circumstances under which a particular deposit was formed. But he cannot be sure in regard to this, from the examination of one or two shells only; since there are many genera, which contain species of both kinds. He is guided, therefore, by the comparison of all the shells contained in the deposit, with their nearest allies amongst those now existing. Sometimes there is such a mixture of marine and fresh-water shells, as to induce the belief, that the deposit was formed in the estuary at the mouth of a river, of which both might be inhabitants at once. In other cases, the shells are so exclusively fresh-water, as to indicate that the deposit was formed at the bottom of a river or lake; and in this case, as might have been expected, it is usually of no great extent. When the nature of the shells indicates a deposit from the bottom of the sea, the same shells are frequently found in strata, which differ greatly in their mineral materials, and which present themselves at very different parts of the earth's surface; and they thus afford important assistance to the Geologist, in determining the real correspondence between these deposits. It is a curious fact, that in all the earlier rocks, down to the chalkformation, the remains of the carnivorous Gasteropods bear a very small proportion to those of the herbivorous group; and their place would seem to have been then supplied by the numerous Cephalopods of predaceous habits, which then infested the seas (§ 896). Nearly all of these disappeared after the Chalk was formed; and the proportion of the carnivorous Gasteropods exhibits a remarkable increase from that period.

CHAPTER XVIII.

OF THE LAMELLIBRANCHIATE CONCHIFERA.

930. THIS group is nearly synonymous with that of Bivalves in the Linnæan arrangement, since all the animals which construct bivalve shells belong to it; but it also contains a few species whose shells are Incottivalve; and some others, in which there appears at first sight an entire departure from the usual form. The Mollusks belonging to this Class are, in common with the Tunicata, destitute of a head; that is, the mouth is not situated upon a prominent part of the body, nor assisted in its choice of food by organs of special sensation in its neighbourhood; but the entrance to the stomach is buried between the folds of the mantle.

931. The part of the structure of these animals which is best known, is the shell. This is composed of particles of carbonate of lime, exuded from the surface of the mantle, and contained in the cavities of cells, or between layers of membrane (§ 870). If one of the valves of an Oyster be examined, it will be seen to consist of a number of layers, of which the external one is the smallest, each inner one projecting beyond the one which covers it. This is the case with other Bivalves; but it is more evident in such shells as that of the Oyster, in which the layers adhere loosely together, than in others in which they are more compact. The shelly matter is thrown out at intervals from the surface of the mantle; and as the animal enlarges at each interval, the new layer extends beyond the old one. In this manner a constant relation is preserved between the size of the animal and that of its shell; and the addition of the newly-formed portions, not to the edge only, but to the interior of the whole previous shell, strengthens the latter in proportion to its increase in size.

932. The valves are connected together in various ways.