Animal Kingdom V.3

Page 26

18 CRUSTACEA. ported by a cartilaginous skeleton, is armed internally with five bony.and notched appendages, which completes the tri­t11ration of the aliment. In it, in the moulting season, which arrives near the end of spring, we observe two calcareous bo. dies, round on one side and flat on the other, commonly called crabs' eyes, that disappear after the change is completed, thereby inducing us to believe that they furnish the material for the renewal of the shell. The liver consists of two large clusters of blind vessels, filled with a bilious humour, which they pour into the intestine, near the pylorus. The alimen­tary canal is short and straight. The flanks present a range of holes situated immediately at the insertion of the branchire, but which can only be seen by removing those organs. The under shell, viewed internally, at least in several large spe­cies, exhibits transverse cells formed by crustaceous laminre, and separated in their middle by a longitudinal range of the same nature. The sexual organs of the male are situated near the origin of the two posterior feet. Two articulated pieces) of a solid consistence, and resembling horns, stylets, or setaceous an· tennre, placed at the junction of the tail with the thorax and replacing the first pair of subcaudal appendages, are regarded as the male organs of copulation, or at least as their sheaths. But, according to our observations on various Decapoda, each of them consists of a little membranous body, sometimes seta­ceous, and at others filiform or cylindrical, that projects from a hole situated at the articulation of the hip of the two poste· rior feet, with the lower shell. The two vulvre are placed on this piece, between those of the third pair, or on their first joint, a disposition depending on the widening and narrowing of the lower shell. Copulation takes place, ventre a ventre, These animals grow but slowly, and live a long time. It is among them that we find the largest and most useful species, but their flesh is not easily digested. The body of some Pali­n uri attains the length of a metre. Their claws are efficacious weapons, and have such power in large individuals, that they have been seen to seize a Goat, and drag it from the shore. DECAPOD A. 19 They usually inhabit water, but do not instantly pc~·isl~ when deprived of it; some species even pass a. part of their hves on land, only visiting the water in the nuptml s~ason, and for the purpose of depositing their spawn. Eve~ they are compelled to fix their domicil either in burrows, or m cool, damp 11laces. The Decapoda are voracious and carnivorous. Certain spe­cies even penetrate into cemeteries, and devout· the dead. Their limbs are regenerated with surp1'ising promptitude, but it is requisite that the fracture be at the junction of the arti · culations and when accident determines it otherwise, they ' . know how to apply a remedy. When they w1sh to change their skin, they seek a retired and solitary spot, in order to be sheltered from their enemies, and to remain at rest. When the change is effected ' their body is soft, and has a more ex.· qnisite flavour. A chemical analysis of the old shell proves It to be formed of the carbonate and phosphate of lime, united in di[erent proportions with gelatine. On these proportions depends the solidity of the s~cll: it is muc.h less thic~ and flexi­ble in the latter genera of tins order, and further on, It becomes almost membranous. M. de Blainville has observed that the shell of the Palinurus is composed of four superincumbent lay­ers' the SUJJerior and two inferjor of which are membranous; (' . the calcareous matter is interposed between them, 10rmwg the fourth. Exposed to heat, the epidermis becomes of a more or less vivid red, the colouring principle being decomposed by boiling water; other combinations of this principle produce, in some species, a very agreeable mixture of colours, that fre· fluently border on blue or green. The greater number of fossil Crustacea hitherto discovered belongs to the order of the Decapoda. Among those of Eu­rope, the oldest approach to species now living in the vicinity of the tropics; the others, or more modern ones, are closely allied with the living species of Europe. The fossil Crustacea of the tropical regions, however, appear to me to bear the closest similitude to several of those now found there in a liv­ing state, a. fact of much interest to the geologist, should the study of the fossil shells of those countries, coliected from the deepest strata, furnish a similar result.