It is, however, interesting to observe that in the most highly organized of the CR CSTACEA NS, the Thwrhylira, the complete centralization of the body and nervous ganglia is effected, as in the case of Insects, in a slow and gradual manner, and that their muscular system under goes a metamorphosis scarcely less remarkable than that observed among the Insects them selves. The Crab on first leaving the egg is almost in the condition of a long-tailed Shrimp, and the locomotive limbs scarcely to be recog nised as being worthy of such a title, being not only rudimentary in their size but exclusively adapted for swimming; and it is only after several times casting its shell and passing through distinct gradations of form, that the muscles of the legs attain the preponderancy over those of the trunk and become strong enough for progression on land.
The fourth grand division of the animal kingdom, comprising the MOLLUSCA of Cuvier, is characterized by the dispersed con dition of the nervous ganglia, which, through out the extensive series of creatures constructed according to this type, are distributed without any symmetrical arrangement in different parts of the body, whence the Molluscs have been ; named by Professor Grant CYCLO-GA NUL' ATA, and more recently by Professor Owen Iliman GA N G LI ATA, the latter term being, as we con ceive, the preferable of the two. In the Mol lusea the general outline of the body partici pates, more or less, in the want of symmetry that is so conspicuous in the disposition of the ganglia composing the nervous system, and the muscular apparatus does not exhibit that pre cision and regularity which is visible among all the Articulata. There is no longer, in fact, any trame-work, but when a shell is present, as, for example, in the Snails and kindred furors, both terrestrial and marine, it is on'y in those parts of the body that are protrusible from the testacenus covering that the tegument exhibitst this decided muscularity, the mantle lining the I shell being constantly thin and membranous.
But the most strongly developed part of the muscular covering of a gasteropod is the broad fleshy disc attached to the ventral surface of the body which constitutes the apparatus of locomotion, and gives the name conferred by zoologists upon the entire class. This disc, or foot, as it is likewise called, is entirely made up of contractile fibres, disposed in various directions, so as to confer all the capabilities of movement necessary for securing progres sion along the plane surfaces over which these sluggish animals are destined to crawl.
Having, as yet, no internal skeleton deve loped, and being equally destitute of any thing like an external articulated frame-work, it must be evident that if creatures of this description are to he provided with organs requiring to be moved by subordinate sets of muscles, seeing that there is no firm point of attachment to be found, as is the case among Insects or the Vertebrata, recourse must be had to a new plan, and accordingly few more remarkable deviations from what is generally met with in other animals can be pointed out than we meet with in the extensive class under consideration. The pints of the mouth, the tentacles, the eyes, and those parts of the male generative system needed for copulation, are in many instances so constructed that they may, when not in use, be completely retracted into the general cavity of the body and packed up amongst the viscera by means of a mechanism quite peculiar, and of which a particular account is elsewhere given. (See GASTLROPODA.) In the PrutoPon MoLtVSCA, we likewise find the entire body enclosed in a muscular bag, forming what is called the visceral sac, but the locomotive organs present themselves under a different aspect. These consist of two muscular flaps or wings, appended to the op posite sides of the neck, which form, in fact, two oars or paddles, wherewith the little Ptero pods row themselves about from place to place, or gambol gaily among the waves, which some times, in the northern oceans, swarm with count less multitudes of them. The lateral fins in ques tion were regarded by Cuvier as being likewise subservient to respiration, an opinion, however, which Eschrieht satisfactorily confutes. The lat ter writer, moreover, points out a little circum stance worth recording, namely, that the wings are not distinct and separate organs as at first they would appear, but that the muscles mov ing them pass continuously in a crucial di rection through the neck of the animal from one wing to the opposite, so as to convert the whole apparatus into an exact representation of the double paddle used by the Greenlander, in rowing his kujac, or canoe, over the very seas frequented by the Pteropods, in such abun dance.