750 



CRUSTACEA. 



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 antennae, 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 

 branchise, 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 oody, 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- 

 chise, and possessing a vascular system like the 

 Crustacea, breathe by means of tracheae, 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 tracheae 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 which 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. 



* See Histoire Naturelle des Crustaces, par M. 

 Milne Edwards, vol. i. p. 231. 



