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 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 a11 the whole superior part of the line of union, (see the traces of it in the thorax of a pholque, pholcus rivulatus, fig. 79.) But in every case the head (a) is recog nizable by constant characters: it supports the eyes and all the pieces belonging to the oral apparatus, while the thorax (b), on the contrary, gives 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 maxillec ; 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 tron is distinctly shewn in fig. 100, e, which represents the inferior surface of the body of the house-spider, (tegenaria domestica.) On the upper surface of the chest we find another plate much more extended than the sternum, and joined anteriorly with the head by means of a fissure or triangular V-shaped notch which receives it. This plate or dorsal shield exhibits divisions or rather lines of suture which the eye readily distinguishes. They represent arcs of circles arising from the base of the legs and all ending in the centre of the thorax, where there is a depression varying as to extent and depth according to the individual. In other arachnidans this structure is not so clearly shewn on account of the close union of the different pieces; but it is easy to detect or at least explain the un important modifications which obtain in these cases. In the figure, which we have taken from Savigny, of the pholcus rivulatus, the traces of the division may be readily followed, (fig. 79, b.) Continued comparative researches have convinced me that this dorsal plate of the thorax of the araneidx is formed, not of the dorsal pieces of the thorax of insects, but only of the lateral pieces or those of the flancs. For the arachnidans being deprived of wings, the intermediate thoracic element or tergum, so largely developed on account of the pre sence of those organs in the thorax of insects, being no longer necessary, has completely dis appeared. How has this taken place ? The flancs (pleura) which in insects were diva ricated and pushed to the sides by the tergum, when that obstacle was removed, have mutu ally approximated and become united toge ther in the middle line, precisely at the place where the little depression exists which we have already mentioned.