« AnteriorContinuar »
and fleshy ventral disc, and endowed with this locomotive apparatus, exhibit senses of proportionate perfection. Lastly, the CEPHALOPODA,* the most active and highly organized of this large and important division of animated nature, are furnished with both eyes and ears, and armed with formidable means of destroying prey, so that they are thus enabled to become tyrants of the deep, and gradually conduct to the most exalted type of animal existence.†
Of these classes of Mollusca, most of them seem to have been introduced at the very commencement of the existence of our globe: but two groups have since then greatly diminished in number and relative importance, although each is still represented in our own seas: these are the Brachiopoda, and the Cephalopoda. The Conchifera and Gasteropoda, or ordinary bivalve and univalve molluscs, seem to have been at first either totally absent or extremely rare; and, although they afterwards increased, they do not seem to have taken the place of the Brachiopoda till after the close of the second great epoch; while the Cirrhopoda and Tunicata may indeed have existed, but have left no remains of their existence in the ancient rocks.
The Brachiopoda were unquestionably the chief and almost the only representatives in the primæval seas of that large class of animals inhabiting bivalve shells, which were then scantily distributed, but now perform an important part in the great world of waters. They exhibit, however, an internal organi
Kepaλn (cephale), the head; woda (poda), feet.
zation extremely simple compared with that of the other bivalves, together with a complexity in the structure of the shell, and in the contrivances for keeping the two valves partly asunder, which are quite peculiar to them. The shell, too, is generally laminated, or even fibrous in its texture: numerous long hairs seem to have passed, in some cases, from the plates, while in others they passed through the plates of which the shell is made up. Nothing can well be imagined more varied than the contrivances by which the ancient species of this group were enabled to obtain existence.
The two valves of brachiopodous shells are not connected by any hinge; but the lower valve was either directly fastened to a rock or some marine substance, or a bundle of fibres or hairs passed through one valve from the other, and were collected into a pedicle or foot-stalk, by which the animal could attach itself at will. Two arms, or tentaculæ, were wound in a spiral within the shell when at rest, but were capable of being expanded in search of food; and these being covered with cilia-those peculiar hair-like appendages frequently met with in animals of imperfect organization-powerful currents were produced in the surrounding water, which being directed towards the mouth as a focus, would hurry into that aperture whatever nutritive particles might chance to be in the vicinity.* The food of these creatures consisted probably of the minutely divided and decomposed particles of dead animals of various kinds floating about in the seas; and different species seem to have been enabled to live at various depths, varying from a
* Rymer Jones's Animal Kingdom, p. 365.
few fathoms to the deepest abysses at which any appearance of life is met with.
The annexed figure (11) represents a common and characteristic silurian species, of the kind afterwards most common; while the figures 12, 13, serve to illustrate the very remarkable internal partition, separating the interior of the shell into several parts in the case represented, and in other instances affording very singular modifications of this curious principle of structure. All the shells of animals of this group have projecting plates of shell, more or less prominent, passing up from the centre of the larger or upper valve. (See fig. 12.)
The Pteropoda of the silurian formations were probably numerous and powerful, attaining a far larger size than they have done at any subsequent period. Several species of a genus (Creseis) still represented by some small Mediterranean species have been determined from some of silurian rocks;
but of the habits of the animal we are not able to speak with any certainty. It is not unlikely, however, that they were exceedingly carnivorous, and supplied the place of the common tribes of univalve shells of after times.* They seem, also, to have preceded, in some measure, the cephalopods, and may, therefore, have performed the same part as the animals of this fierce and powerful group.
Of the true Cephalopoda there are several genera described from the rocks of the oldest period, and they differ chiefly in the shape of the singular manychambered habitation, which is, in fact, the only part left by which we can identify these animals. They all bore a much greater resemblance to the nautilus than the cuttle-fish, and in this respect seem to exhibit the same peculiarity that has been already so often alluded to, namely, the usual introduction of groups of species possessing the lower organization of their tribe in the earliest formed strata of the earth.
The Nautilus (see fig. 52), the lowest existing type of the Cephalopoda, (which, however, it will be remembered, form the highest division of the Mollusca,) exhibits a great advance in the construction of the organs of animal life, by which it is readily distinguished from the ordinary inhabitants of univalve shells. In the first place, this animal has a true
* The Gasteropoda themselves were not unrepresented in the seas of the earlier epoch, although they do not appear in the lowest rocks of all. There are at present sixty-three silurian species known-a number scarcely exceeding that of the Orthoceratites from the beds of the same age; while in the proportion of individuals whose fragments are found there is no comparison, the latter being far more numerous.
internal skeleton, and a perfect symmetry throughout the animal and vital organs. The muscular system also forms a larger proportion of the body; the nervous centres, concentrated in the head, have received a marked increase of bulk; the organs of the external senses are much more perfectly developed, and the respiratory tube has received an enormous development, and assists in propelling the cephalopod through the The organs of locomotion and prehension are now arranged round the aperture of the mouth, which besides these possesses jaws working like the beak of a bird, and a strong spiny tongue. The organs of locomotion and prehension are, however, exceedingly simple and very numerous, differing in this respect from the more highly organized cuttle-fish.
Lastly, in the shell we see a marked approach to a higher form of animal existence than is exhibited in other univalve shells. In the few animals inclosed in shells that are able to swim, we find the shell of very diminutive size, of simple form and structure, and of an extremely light and delicate texture. In the nautilus, on the other hand, we find a large, powerful, and complicated shell, composed of a number of separate compartments or air-chambers, all of them together forming a float, and enabling the strong and muscular occupant to rise at will to the surface of the water, or sink down into the depths of the ocean in search of the food of which it no doubt requires an abundant supply.
It is probable that the nautilus and its shell together are somewhat, though very little, heavier than water, when the animal has retired completely within its habitation. When, however, it expands