Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/291

ARACHNIDA

The Arachnida, or, popularly speaking, the Spiders and their allied forms—from ἀράχνη, a spider, and εἶδος, the form or outward appearance—constitute a well-defined group within the great branch "Articulata" of the animal kingdom, though on one side its boundary lines melt almost insensibly into the Annelides, through the family of acarids—Pentastomides. Linnæus and others of the older systematists included all Arachnids (as then recognised) under the class Insecta, Lamarck first separating them into a distinct class. A few general remarks, therefore, on the branch Articulata, and its subordinate groups, may be useful, to show the general relations of that under consideration.

As distinguished from the Vertebrata, the animals of the branch Articulata may be characterised generally as having (besides the marked and uniformly different position of the chief organ of circulation and the nervous chords) an external skeleton, the different parts of which, of more or less solid consistency, are articulated, or jointed to each other, and contain within their hollows the vital and other organs. Internally, threads of nervous matter, springing from enlargements or knots (ganglia) upon two or more central longitudinal nervous chords on the ventral side of the body traverse the different parts of it; these chords, however, end in no one special mass, corresponding to the brain in the Vertebrata; and the nervous systems composed by these ganglia vary considerably in their special structure and arrangement in the different subordinate groups.

This being the distinctive plan of structure in the Articulata, a review of the whole group shows us that this plan is carried out in two well-marked leading ways—first, without, and secondly, with, articulated legs, or organs of locomotion and touch. Those without articulated limbs constitute the Annelides of Lamarck, which, for our present purpose, it is sufficient to characterise as having the articulate plan of structure carried out in the general form of a more or less cylindrical, lengthened body, composed of a number of rings, segments, or transverse folds, of which the first forms the head, but destitute of articulated limbs. Those with articulated legs constitute the Condylopa of Latreille, or the Insecta of Linnæus; but the term Insecta being now in general use to define a subordinate group, it will be better to adopt here the name given by Latreille. In the "Condylopa," then, the articulated plan of structure is carried out in the form of a more or less hard body composed of different pieces, of which one, two, three, or more, are generally leading ones; to this body (in general to the anterior portion of it) are attached articulate organs of locomotion various in number, but never, or at least very rarely, less than six. The organs of sight (when present) are always in the head, and vary in number, position, and kind. To these two leading divisions of the branch Articulata—the Annelides of Lamarck, and the Condylopa of Latreille—we may give the designation of Classes, since they are each respectively composed of animals which thus primarily and markedly differ in the way in which their common articulate plan of structure is carried out.

Leaving now the class Annelides, to which has been given the name "Anarthropoda," and confining our attention to the "Condylopa," or "Condylopoda" (equivalent to the "Arthropoda" of more modern authors), we find that the latter contains the four following groups—"Insecta," "Myriapoda," "Arachnida," and "Crustacea."

These all agree in the way in which, as above explained, their general articulated plan of structure is carried out, but they differ in the further modification of this plan, that is to say, in the number, division, and size (both absolute and relative) of the different articulations of the body (including the organs of locomotion, of manducation, and of touch), and in the respective value and relation of these to each other, as well as in the modifications of their respiratory and other internal systems, and also in other important points. Differing thus from each other, these four groups, usually (three at least of them) ranked as distinct classes, yet appear to be entitled only to the rank of sub-classes, being each merely an equally subordinate group of the class "Condylopoda."

The four sub-classes of Condylopoda (or articulate-legged Annulosa) may be separately and briefly characterised thus:—

Sub-class I.—Insecta: subject in general to metamorphosis, more or less complete, during the progress to maturity; organs of locomotion, six; body divided into three principal parts—caput, thorax, and abdomen; respiration wholly tracheal.

Sub-class II.—Myriapoda: not subject to metamorphosis, properly so-called; organs of locomotion above six in number (twenty-four and more) developed by degrees, and affixed to the abdominal as well as to the other (numerous) segments into which the body is divided. Although occasionally some of the anterior segments coalesce, and the head is stated to be formed by five or more modified segments, yet no one or more parts of the body can be said to be principal ones; respiration is tracheal.

Sub-class III. Arachnida: not subject to metamorphosis, properly so-called, though, as with the Myriapoda, some portions of structure among certain acarids, and to a small extent even among the Araneidea, are progressively developed at the various moultings of the skin. Organs of locomotion, eight; body in general divided into two principal parts, cephalo-thorax and abdomen; but where (as in some groups) the abdomen is divided into more or less distinct segments no organs of locomotion are ever attached to them; respiration, where distinct organs exist, is either tracheal, pulmo-branchial, or the two coexistent.

Sub-class IV.—Crustacea: without metamorphosis; organs of locomotion various in number (not less than eight or ten), and attached to some or all of the many segments, abdominal as well as others, into which the body is divided; although in numbers of crustaceans no one part of the body can be called a principal part, yet in some very large and leading groups, the portion covered by the carapace (corresponding to the cephalo-thorax in Arachnids) may well be termed the one principal part; the abdomen being in this case more or less rudimentary or subordinate. Respiration in the Crustacea, where distinct organs are present, is branchial.

In this view the following diagram shows the relative positions of the several leading groups of the Articulata:—