ARACHNIDA; oteavv, aranea ; Eng. arachnidans ; Fr. arachnides ; Germ. Spinne ; Ital. Ragni.
This class of animals was for a long time confounded with that of insects, but it has been distinguished therefrom by many modern natu ralists, and more especially by Lamarck, who has applied to it the term arachnides,' now universally adopted.
The characters indeed which the arachnidans present are perfectly distinct, and prevent them from being confounded either with crustaceans or insects, although one cannot avoid perceiving that they have numerous relations with the animals of these two classes, and they are con sequently placed in natural arrangements be tween the crustaceans and insects.
Zoologists have assigned the following cha racters as peculiar to and distinguishing this class.
Body divided into thorax and abdomen ; apterous. Legs, eight in the adult state. Head continuous with the chest. Eyes smooth. Sex ual orces situated either on the thorax or base of the abdomen.
To these external characters may be added others derived from the anatomical conditions of different organs. Thus all arachnidans pos sess exclusively an aerial respiration, either effected by a sort of lungs, or by means of tracheal tubes, as in insects. This difference in the respiratory organs is accompanied with one not less marked in those of circulation ; for example, all the pulmonary arachnidans possess vessels which carry blood, while, on the contrary, all those which have tracheae are deprived of bloodvessels. Lastly, the latter species (or trachearies) alone undergo metamor phoses analogous, in some respects, to those of insects ; while the former (or pulmonaries) suffer only changes of integument. We shall treat further on these peculiarities hereafter.
Our object here not being to treat of classifi cation, we shall refer the reader for this subject to the works of Cuvier, Leach, Latreille, Walcknaer, Duges, and limit ourselves at pre sent to a tabular exposition of the principal divisions and subdivisions admitted in this class down to the genera with which it is most essential to be acquainted.
Latreille, whose method is that most gene= rally adopted by zoologists of every country, divides the arachnidans into two great orders, as follows :— M. Walcknaer, who has made a special
study of the family of araneidee or spinning arachnida, and who has published many works on their methodical distribution and their habits of life, has very recently considered them with the express view of arriving at a natural arrange ment of them ; the result of his labour may be seen in the following Of the external covering or tegumentary system.—Although the external covering of the arachnidans varies in consistence, according to the part of the body which is examined, yet it may be said in general to be more or less soft, rarely acquiring the solidity of the integument of certain insects, and still less the hardness of that of many crustaceans.* Where it is of the greatest consistency it is elastic, of a deep brown colour, and has an aspect analogous to horn. In chemical com position, however, it is always widely different, as has been proved by the researches of M. August Odier, and some other chemists. It contains, in fact, a substance sui generis, called chitine,' which is insoluble in potassa, but, on the contrary, is soluble in warm sul phuric acid, does not turn yellow with nitric acid, and does not curl up when burnt, but leaves an ash, which, if the part experimented on is sufficiently thick, preserves the form of the organ.
The solidity of the outer covering is gene rally greater on the thorax than on the abdo men. The genera scorpio, phrynus, theli phonus, and phalangium, afford an exception to this rule, the rings of the abdomen being distinct and solid, especially on the dorsal aspect.
In the spiders properly so called, (aranete,) and in the greater number of the mites (acari), the skin of the abdomen is very soft, coria ceous, papiraceous, or even membranous, transparent, and susceptible sometimes of be ing greatly extended. It is on this account that the abdominal segment of the body shrinks and loses its form after death, and from the transparency of the integuments the same arachnidans present during lifetime the various markings and lively colours which depend on a kind of pigment situated in the interior of the body.