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Show 158 CRUSTACEA. For a description of the species and their localities, we refer the reader to the excellent work of this celebrated naturalist, who in hia labours upon the fossil Crustacea, properly so called, or universally admitted as such, has availed himself of the talents of one of his most distinguished pupils, M. J?esma~est, frequently refe~red to by us, not only with respect to thlB particular part of the SClence, but in relation to his work on the living Crustacea. DifFerent naturaliata have proposed various generic sections of these fossils; but being restricted to general considerations, I have adopted those presented to ua by the best work hitherto produced on the subject. 159 CLASS II. ARACHNIDES. The Arachnides, which compose the second class of articu· 1ated animals provided with movable feet, are, as well as the Crustacea, deprived of wings, are not subject to changes of form or do not experience any metamorphosis·, simply casting their skin. Their sexual organs also are at a distance from the posterior extremity of the body, and situated at the base of the abdomen, those of several males excepted: but they diifer from them as well as from Insects in several particulars. Like the latter, the surface of their body presents apertures or transverse fissures called stigmata(!), for the introduction of air, but they are few in number-eight at most, and usually but two-and confined to the inferior portion of the abdomen. Respiration is also effected either by means of air-branchire, fulfilling the functions of lungs, that are contained in sacs of which these stigmata are the apertures, or by radiated trachere( 2). The visual organs merely consist of ocelli, which, when numerous, are variously grouped. The head, usually confounded with the thorax, in place of the antennre, has two articulated pieces in the form of small didactyle or mo~ nodactyle chelre, improperly com pared to the mandibles of Insects, and so denominated, moving in a contrary direction to the former, or from above downwards, still however cooperating in the business of manducation, and replaced in (1) A vague and improper appellation, for which '\\•e might sub11titute pnt11f1&0$• lollaa,-air-mouth,--or apiraculum. (2) See general observations on Insect1. |
