CRUSTACEA. Eng. Crustaceans ; Germ. Krustenthiere; Fr. Crustaces —This is the name given to a class of articulated animals, the type of which we have in the common crab and lobster, and which is essentially distin guished by the conformation of the organs of circulation, of respiration, and of locomotion.
The body of these animals is articulated ; that is to say, it is divided into rings, for the most part very distinct and partially move able; their integuments are of considerable consistency, being either horny or calcareous, and form a kind of external skeleton; their extremities are also articulated, arranged in a double series, and constitute antennw, jaws, limbs, (ambulatory, natatory, or prehensile, the most common number of which is five or seven pairs,) and other appendages ; their nervous system is ganglionic, situated partly in front of the alimentary canal, and partly behind and below the intestine; their blood is colourless, and put into motion by an aortic and dorsal heart; their respiration is. almost invariably aquatic, and is accomplished by means of branchiw, or the skin only; to conclude, the sexes are distinct, and the organs of genera tion double.
Great and striking analogies occur between the Crustacea, the Insecta, and the Arachnida; so that it was long the custom to associate the whole of the animals now comprised in these three classes, under the single name of IN SECTA, or Insects. Brisson and Lefranc de Berkhey proposed, it is true, to separate the Crustacea, but the classifications of these writers not being based upon organic charac ters of sufficient consequence, did not receive the general assent of naturalists, and it is only since the beginning of the present century that the necessity of separating the annulosa into certain distinct classes has been universally acknowledged. This result was mainly due to the anatomical inquiries of Cuvier, and this great naturalist was even the first who established a class among the invertebrate series of animals for the reception of those having bloodvessels, a ganglionic spinal cord, and articulated extremities, characters which, at the present time, still suffice to distinguish the Crustacea from the greater number of other animals.
It is more especially in the general confor mation of the body, in the structure of the extremities, and in the organization of the ner vous system, that the Crustacea resemble the Insects and Arachnidans. The apparatus of vegetative life in these different animals pre sents numerous and important differences. Thus Insects, instead of breathing by means of bran chim, and possessing a vascular system like the Crustacea, breathe by means of trachew, and have no bloodvessels; and Arachnidans, which, like the Crustacea, have a heart more or less perfect, and distinct vessels for the circulation of their blood, have an aerial respiration ef fected either by the medium of trachew or of pulmonary sacs.
The whole of the Crustacea are evidently formed after one and the same general type ; still, numerous and extensive varieties of struc ture are observed among these animals; and when compared one with another, their orga nization is found to become more and more complicated in proportion as we rise in the series comprised by the group; it is farther found that the lower links of this kind of chain re present, to a certain extent, the different phases through vvhich the more perfect Crustaceans pass during the period of their embryonic ex istence.
This diversity of organization affords the grounds by which naturalists are guided in their distribution of the Crustaceans into orders and families.
The natural arrangements of these animals that have been followed, are consequently ob served to vary with the extent of knowledge of their structure possessed. It were tedious to enter upon the consideration of the different sys tems which have been successively proposed for their classification; in order to aid the mind in the comprehension of the anatomical details into which we shall have to enter in the course of this article, it will be enough for us to pre sent at once those divisions which appear to indicate most truly the differences and resem blances subsisting between the various mem bers of the class ;* and to do this in the most compendious manner, and to exhibit the clas sification which thence ensues, we shall present them to the reader in the shape of a synoptical table.