CLASS CRUSTACEA.
The arrangement of the Crustacea in systems of natural history has undergone numerous changes; they were placed by Linnæus among the insects:—others considered them to have more analogy to the spiders. Lamarck was the first who made the Crustacea an independentCLASSforming twoorders, which he has named after the arrangement of the branchiæ, or gills, by which they breathe, namely,—theCrustacea homobranchiæ, the distinguishing marks of which he describes in this manner: branchiæ hidden under the lateral margins of a kind of cuirass, covering the body of the animal, with the exception of the tail; the mandibles always furnished with feelers, the eyes placed on footstalks, the head not distinct from the trunk, and possessing ten feet to assist them in their movements. Secondly, theCrustacea heterobranchiæ, in which the branchiæ are external, in various situations, but never under the lateral margin of a cuirass; they are either under the belly or the tail, adhering to the feet, or confounded with them: the eyes are in general fixed,sedentary, not on footstalks.
The first of these orders, thehomobranchialCrustacea, includes most of the larger kinds of shell-fish, as, for instance, crabs, lobsters, and cray-fish; shrimps and prawns are also in this division.
Their organization is much more perfect than that of the other order, and, according to Lamarck, it is among these animals that the lastappearance of the organ of hearing is seen, in tracing the animal kingdom from the most perfect animals to those whose formation is apparently less complex.
The body of these creatures appears to be composed of only two principal parts, the body and the tail; for the head is so intimately united and confounded with the trunk, as to appear to be merely a portion of it. The two eyes are fixed at the top of two moveable supports, and are placed in a hollow prepared for their reception, on each side of a projecting portion of the shell that covers the head. The antennæ, which are usually four in number, are placed about this spot; they are inserted beneath the stems that support the eyes. The two outermost of these antennæ are generally the longest.
The branchiæ, or organs by which they breathe, assume a form somewhat pyramidical, arranged like a series of leaves, or the web of a feather; they are placed in the interior of the shell along each side, and are so arranged as to adhere to the roots of the feet, so that each of these feet has a hidden branchia attached to its base.
The mouth is composed of a fleshy lip, projecting between the mandibles; of two hard triangular mandibles, more or less notched at their extremity, and each having a kind of feeler inserted on the upper part; they also possess a little tongue between these mandibles, at the root of which is the opening to the stomach: they have besides two pair of jaws, like leaves, the borders of which are fringed, and six other members, which Lamarck callsfoot-jaws, from their bearing some resemblance to legs, or feet. From this it appears that the parts of the mouth in the Crustacea form a complicated apparatus, and accordingly we find the whole tribe exceedingly voracious, the Crabs inparticular, feeding upon any animal substance, putrid or not, that may come within their reach. Some of the species are well known as articles of food, but they are not equally wholesome at all seasons of the year.