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Show 2 CRUSTACEA, AH.ACHNIDES, INSECTA. and which is composed of two pieces(l ), is called ~he c~x~, or hip; the following one which is, usually, ne~rly Jn a ortzontal position, the femur, or thigh; and the thir~, generally vertical, the tibia or leg. To these ensues a smte of small ones which touch the ground, forming the true foot, or what is denominated the tarsus. The hardness of the calcareous or horny(2) envelope of the · 1· · wing to that of the ex-greater num bcr of these amma s, JS o . . cretion, which is interposed between the de~m1s and e~Ider-mis, or what is termed in man the mucous t~ssue. . This ~xcretion al~o contains the brilliant and varied colours With whiCh they are so often decorated. . They are always furnished with eyes, whiCh are of tw.o kinds: simple or smooth eyes(3), which resemble a v.ery m~nute lens, generally three in number, and arranged 1n a triangle on the summit of the head; and co~pound eyes, where the surface is divided into an infinitude of different lenses called facets, to each of which there is a correspon?ing fila~ent of the optic nerve. These two kinds may be etthe: umte~ or separated, according to the genera. Whether then' functwns be essentially different in those cases where. they are found to exist simultaneously, is a problem that remains to be solved; but vision is effected in both of them by means differing widely from those which produce it in the eye of the Vertebrata( 4 ). (1) In many of the Crustacea the second portion of the coxa see~s. to form part of the thighs. The tibia, as in the Ar:lchnides, is divided into two JOmts. (2) According to M. Aug. O<lier, Mem. de la Soc. d' Hist. Nat.,. 1823, t. I, p. 2? et seq., the substance of this envelope is of a peculiar nature, w luch he calls Cht· ( tine. He states that the phosphate of lime forms the great mass of all the sal~s contained in the teguments of Insects, while that in the shell of the Crustacea ts but trifling, though it abounds in the carbonate, which !s not f?und in the prece· ding animals. Other observations, those of M. Straus m particular, demonstrate that the teguments here replace the skin of the Vertebrata, or that they do ~ot form a true skeleton. Those of M. Odier also militate against all the analogtes attempted upon this subject. (3) Ocelli stemmata. ( 4) See the Memoir of Marcel de Serres on the simple and com~ot~nd eyes of Insects, Mantpellier, 1815, Svo. Also the observations of M. de Blamvllle on t~e eyes of the Crustacea, Bullet. de la Soc. Philomatique. We shall return to this 11ubject at another period. CRUSTACEA, ARACHNIDES, INSECTA. 3 Other or·gans which for the .first time are here presented to us, and which are found in two of these classes, the Crustacea and the Insecta( I), the antennm, are articulated filaments, varying greatly in form, and frequently according to the sex, attached to the head, appearing to be peculiarly devoted to a delicate sense of touch, and perhaps to some other kind of sensation of which we have no idea, but which may refer to the state of the atmosphere. These animals enjoy the sense of smell and that of hearing. Some authors place the seat of the first in the antenn::e(2), others, M. Dumeril for instance, in the orifices of the trache::e, and Marcel de Serres, &c. in the pal pi; neither of these opinions, however, arc corroborated by positive and conclusive facts. As to the second, it is only in the Crustacea Decapoda and some few of the Orthoptera, that we can find a visible ear. The mouth of these animals presents a great analogy, which, according to Savigny(3), and at least with respect to the Hexapoda, extends to those which can only feed by the suction of liquid aliment. . Those called Tritores or Grinders (b1·oyeurs), on account of their having jaws fitted for triturating their food, always present them in lateral pairs, placed one before the other; the anterior pair are especially called mandibles; the pieces which (1) And even in the Arachnides, but under different forms, and with different functions. (2) As regards insects, and when they are claviform, or terminate in a club more or less developed, or furnished with numerous hairs. According to M. Robineau Desvoidy, the intermediate antenna: of the Crustacea Decapoda are the olfactory organ, Bullet. des Sc. Nat.; but he adduces no one direct experiment in pro~fofhis opinion. It would, if this were so, seem probable that in the highly ~armvorous C~·ustacea, such as the Gecarcini and others, we should find this organ m a compa:a~vely greater state of development, '~hereas the fact is directly the reverse. Ills 1deas respecting the external composition of the Crustacea Decapoda suppo.se ~he existence of a skeleton. He should have commenced, however, by establis~t~gthe connexion of these animals with the Fishes, and not by admitting, as a posthve fact, what is at least a matter of doubt. (3) Memoire aur lea animau:x: sana vertebres. The original idea was thrown out, but undeveloped, in my Hist. Gen. des Imectea. |