Page images
PDF
EPUB

Yet as the spines are employed by the seaurchin to effect its motions, there must be some intermediate agent, hitherto undiscovered, which it has at its command, by which it can act upon them. Dr. Carus’ remarks on the zoophytes in general are very applicable in the present instance—“ When we find,” says he,“ that there can be respiration without lungs; that nutrition, growth, and secretion may exist without a circulation of fluids; and that generation may take place without distinct sexes, &c. why should we doubt that sensitive life may exist without nerves, or motion without muscular fibres ?” It is important to be observed here, that these spines, however strongly attached they may appear in the living animal, in the dead one fall off upon the slightest touch, which proves that the cause of their adhesion is connected with its life.

But though it is difficult to detect the muscular fibres that move the spines of the common sea-urchin, I had an opportunity, when correcting the proof containing the preceding paragraph, through the kindness of my friend Mr. Owen, of the Hunterian Museum, well known for his admirable anatomical description of the animal of the pearly Nautilus, of examining a preparation of the large spines, with their sacs, of the mammillary Sea-urchin, in which the muscular fibres were distinctly visible, enveloping the base of the spine, when the sac was removed; so that, reasoning from analogy, it may be concluded that the spines of the common species have a similar muscular apparatus.

1 Nautilus Pompilius. 2 Cidaris mamillatus. PLATE III. Fig. 4.

The spines vary much in their form and sculpture. In the species last named they seem to be of a horny substance, varying in magnitude and length, the larger ones tapering from the base and being blunt at the tip, they are beautifully fluted like the shaft of a corinthian pillar. The part enveloped by the membrane before mentioned, is thicker than the rest of the shaft, perfectly smooth, but terminates in a bead: they are tinted with violet, but the base and tip, or the pedestal and capital of the pillar are white. The base is concave so as to play upon the levigated centre of the above protuberance. Besides these larger spines, there are some bristled-shaped ones terminating in a subovate knob, which when unfolded appears> to resemble a tripetalous flower with acuminated petals, and which are supposed to be polypes. Those parts void of spines, called the alleys, distinguished by rows of orifices disposed in pairs, are furnished with a quite different kind of organ, I mean the suckers before alluded to and described, by which the animal can also move

i Cidaris mamillatus, PLATE III. Fig. 14. * Pedicellariæ, Ibid. Fig. 12, 13. 3 Ibid. Fig. 14.

or fix itself to any substance; it is thought also, as they are perforated, that it uses them to absorb the water for respiration. The length of these suckers or tentacles, for so they may be also called, when they are fully extended, is always greater than that of the spines, so that they may serve as so many anchors to fix the animal and enable it to resist the mass of waters that

press upon it. They are stated to be more numerous near the mouth than in other parts, by which arrangement Divine Wisdom has fitted them to maintain a horizontal position, which is their natural one. These suckers fix the animal so firmly to the rocks, that it is with the greatest difficulty, and seldom without crushing the shell, that they can be separated.

The most powerful and complex organs with which the Creator has gifted the Echinidans are their jaws and teeth. Their mouth has adapted to it a remarkable frame-work, consisting of five pieces, corresponding with five segments, into which the shell may be divided ; each of these pieces forms an arch,' and the whole a pyramidal frame, which was compared by Aristotle to a lanthorn without a skin. To these are attached the moveable part of the apparatus, consisting of five jaws, each containing a long tooth, the teeth converging in

'PLATE III. Fig.3. d.

2

Ibid, Fig. 10, 11.

VOL. I.

P

the centre close the mouth. Altogether this complex machine consists of twenty-five pieces moved by thirty-five muscles. The disposition of these pieces, Lamarck observes, and of their moving muscles, indicate that the parts of this machine can have only a common movement, and no one of them an individual or separate one; but it appears from Cuvier's elaborate description of this wonderful and complex machinery, if I understand him right, that the action of certain muscles will give to any one of the teeth that form the pyramids an independent motion. This powerful apparatus, which the animal can incline in different directions, indicates a kind of food, less easy to bruise and masticate than what we have seen satisfies the whale, and these organs afford a singular contrast to those by which that enormous monster masticates its food.

The Echinidans, whose station appears to be often near the shore upon submerged ledges of rock, feed upon whatever animal they can seize. We have seen that they sometimes turn upon their back and sides, as well as move horizontally, this enables them more readily to secure their food, with the aid of the numerous suckers in the vicinity of their mouth, which when once they are fixed, never let go their hold till the animal is brought within the action of their powerful jaws. La

· Plate III. Fig. 9.

marck thinks they do not masticate but only lacerate their food ; but as two faces of each of their pyramidal organs answer those of the two adjoining ones, and these faces are finely and transversely furrowed, this looks like masticating surfaces. Bosc, who appears to have seen them take their food, says it consists principally of young shell-fish, and small crustaceous animals; as the latter are very alert in their motions, it is difficult for the sea-urchins to lay hold of them: but when once one of these animals suffers itself to be touched by one or two of the tentacles of its enemy, it is soon seized by a great number of others, and immediately carried towards the mouth, the apparatus of which developing itself, soon reduces it to a pulp.

Who can say that the All-wise Creator did not foresee all the situations into which this animal would be thrown, so as to provide it with every thing that its station and functions require ? Considering its internal organization and the nature of the animal itself, and that it holds a middle station between the polype and the Molluscans, in the former of which the developement of muscle is very obscure, and in the latter very conspicuous, and that it cannot, like the former, fix itself by its base, and so support a polypary, or if endued with locomotive powers carry with it a heavy shell ; these things con

1 PLATE III. Fig. 11.

« PreviousContinue »