ARACHNIDA. 



201 



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- 

 phojius, 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, (aranea,) 

 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. 



The head, as we have remarked in our ex- 

 position of the characters of the class, is always 

 consolidated with the thorax; this is readily 

 ascertained to be the fact in scorpions and 

 spiders, and in order to express this dispo- 

 sition, which obtains also in many of the 

 Crustacea, the two united segments are termed 

 * cephalo-thorax ;' the term abdomen is applied 

 to the part properly so called, and thus the 

 body of the arachnidans may be divided into 

 two parts. The abdomen may be either sessile 

 or pediculate, i.e. it may either inclose at its 

 anterior margin the posterior part of the thorax, 

 as in the scorpions, or it may adhere to the 

 thorax by a very circumscribed part of that 

 margin, as in the spiders properly so called. 

 Anatomically speaking, the abdomen has a 

 very simple structure : it is formed of annular 

 segments sometimes distinct and hard, as in 

 the scorpions ; sometimes blended together 

 and soft, as in the spiders and mites. 



The other division of the body or cephalo- 

 thorax is not so simple. To facilitate the study 



* This composition being precisely analogous to 

 that of the integuments of insects, we shall treat 

 of it in the article relating to these animals. 



Pholcus rivulatus. 



of this part it is necessary to consider the 

 cephalic portion separately from the thoracic 

 division. This it is easy to do, where, as in 

 many cases, the junction of the two parts is 

 perfectly distinct, and made obvious by the ex- 

 istence of a furrow along all the whole superior 

 part of the line of union, (see the traces of it 

 in the thorax of a pholque, pholcus rivulatus, 

 jig. 79.) But in every case the head () is recog- 

 Fitr 79 nizable by constant 



characters: it supports 

 the eyes and all the 

 pieces belonging to 

 the oral apparatus, 

 while the thorax (6), 

 on the contrary, gives 

 c insertion to the four 

 pairs of legs, which 

 on account of their ex- 

 treme length are repre- 

 sented in the figure as 

 truncated. 



The head is often as narrow as the chest, 

 abruptly truncated anteriorly, and terminated 

 by a point posteriorly, so that it appears by its 

 backward prolongation to separate the right 

 from the left side of the thorax, and to be 

 placed between them like a wedge, (as in 

 the pholcus.) The suture is very close, and 

 sometimes so far effaced that it is no longer 

 possible to decide where the head ter- 

 minates and the chest commences. We 

 have already observed that the head sup- 

 ports the eyes on its upper part, and has the 

 oral instruments attached to its lower surface. 

 These consist, first, of a pair of mandibles or 

 forciples; secondly, of a pair of maxilla:; 

 thirdly, of a sternal labium. 



The number of annuli or segments which 

 enter into the composition of the head of an 

 arachnidan may yet be determined at some 

 future period : we have made some attempts 

 to unravel this subject, but our observations 

 are not yet sufficiently matured to permit us 

 to decide so difficult a question. 



Our researches on the thorax of articulate 

 animals have led to more decisive results, 

 which we shall now expound, but for the 

 complete understanding of which we must 

 refer the reader to the article INSECTA, 

 where a more general theory of the thorax, 

 and a description of all the pieces that enter 

 into its composition will be given. In the 

 arachnidans many of these pieces are entirely 

 wanting; and their thorax is consequently 

 more simple than that of insects : it is- even 

 more simple than the thorax of crustaceans, 

 to which, however, it bears a great resemblance 

 in many points. If, for example, we take a 

 large spider, as a mygale avicularia, and strip 

 off the hairs which clothe the thorax, we shall 

 easily perceive a plate, or plastron, interme- 

 diate to the right and left series of legs. This 

 plastron is the sternum, or, to speak more cor- 

 rectly, the union of several sternums, which, 

 were it not for this union, would manifest 

 themselves as four distinct pieces; that is to 

 say, corresponding in number to the pairs of 

 legs which arise from them. This sternal plas- 



