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 ' τoith articulated legs constitute the Condylopa of Latreille, or the Insecta of Linnaeus; 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— a Insecta f “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 f urther modifications 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— car>ut, 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:— [table]

In this diagram the Myriapoda are placed, in the position ordinarily assigned to them, between the Insects and Arachnids; but perhaps a more natural position would be after the Crustacea, for they appear to connect the Insecta and Crustacea more than they do the Insecta and Arachnida. The point, however, of the special position of any large group in a linear series is tolerably unimportant. A linear arrangement is of course the only one possible on paper, but it cannot express the numerous cross relationships that become evident when the affinities of special groups are closely studied. By placing the Myriapoda after the Crustaceans, we seem to get an ascending series in respect to the organe of locomotion: in the Insecta, six—always attached to the thoracic segments; in the Arachnida, eight—almost always attached to the thoracic segments; in the Crustacea, eight, ten, and upwards—frequently attached to the abdominal as well as to the thoracic segments; in the Myriapoda, twenty-four and more—always attached to the abdominal as well as other segments. The relation of the Myriapoda to the Crustacea through the attachment of limbs to the abdominal segments is thus expressed; and by the meeting of the two ends of the line, the always acknowledged affinity of the former to the Insecta is also expressed.

Having thus briefly explained the most obvious relationships of the Arachnida, let us now proceed to a more detailed, though still general, diagnosis of this group. And first, in respect to their external organisation, Arachnids are articulate animals, with eight articulated legs, each in general consisting of seven joints; the head and thorax are soldered together into one piece (cephalo-thorax), from which with few exceptions the legs invariably spring. In one group of the Arachnida (Solpugidea) the cephalothorax is separately segmented, as it is also in one family of the Thelyphonvdea (Tartarides) ∙, the rest of the body forms also one more or less homogeneous piece (abdomen). The abdomen is always more or less closely attached to the cephalo-thorax; in some instances it is soldered to the thorax, and forms with it one undivided piece, in other cases it is only joined to it by a narrow pedicle. At times, also, the abdomen constitutes a simple unsegmented portion, at other times a segmented or annulate portion of the body, occasionally, as in the true scorpions, prolonged into a segmented tail: in one small but very distinct group tfΓhelyphonidea), the abdomen is prolonged into either a simple button, a short jointed, or a more or less long setiform tail · in another large group (Araneidea) the abdomen terminates with organs (spinners and spinnerets) for spinning threads. The eyes (when present) are simple, always sessile, and placed on the fore part of the cephalothorax; they seem to represent the simple ocelli of the Insecta; the large compound eyes of that order having here no representation, the caput of Arachnids appearing to begin at a point posterior to that which bears the antennae and compound eyes of insects. In Arachnida the number of eyes varies from two to twelve. In front of and articulated beneath the fore part of the cephalo-thorax, and moving in different planes in different groups, are two independent, variously modified, organs for seizing and compressing the insects or other substances on which Arachnids in general prey; these organs are often called mandibles, but more generally, and very appropriately, falces : these are considered by some systematists to be the true homologues of the antennae in insects, and to have been derived from those portions of the insect organisation by long and unceasing modification; when, however, the caput of an Arachnid (scorpion or spider, for instance) is compared with that of a coleopterous or hymenopterous insect, in which the mandibles are well developed, there seems far more reason to conclude that the antennae of the insect have no homologue at all in the Arachnid (Claparède, l.c. post), but that the falces of the latter are the true representatives of the mandibles of the former. Behind the falces (and also used in manducation) are two other large movable portions of structure called maxillae; these vary in shape and size, and form in fact lateral and hinder boundaries to the mouth, as well as an apparatus for comminuting and squeezing the food substances; from each of these maxillae, on the outer side, springs a palpus of four or five joints, varying in structure and use. Between the maxillae most Arachnids have also a fixed piece (labium) of various form, completing the hinder limits of the mouth organs, and in some (perhaps in most) there is, within the parts already named, another portion (tongue), not yet sufficiently observed either in regard to its form or use, but probably acting so as to hinder choking. In some Arachni-dans these different parts of the mouth are soldered together and form a tolerably simple sucking apparatus, analogous to the mouth of some insects (Hemiptera, &c.) As observed above, Arachnids are not, in a proper sense, subject to metamorphosis: in most of them there is little real change after they leave the egg; though, in some of the lowest forms, indeed, there are after-changes by moultings of the skin, which approach the incomplete metamorphoses of some insects.

With respect to the internal organisation of the Arachnida, it might perhaps be enough here to refer to the more detailed accounts given further on, where the separate orders are under consideration; but the following summary will probably make our general view of the whole sub-class more complete.

Muscular System. —Of this it is enough to say that it is similar to that of the rest of the Articulata, consisting of flexor and extensor muscles, situated within the hollows of the limbs, besides groups of fibres by which the epidermis and parts within it are connected and held together. On the outer surface these groups frequently show, at their points of union, boss-like marks or foveae, or impressed spots of various forms and sizes, often presenting by their position distinctive peculiarities of form; it is probable that many others of the external specific.markings are dependent on the course and position of the muscular fibres, as well as of the heart and other organs.

Circulatory System.— The vital fluid is circulated by means of an elongated muscular vessel or heart, varying in form, and, instead of being placed, as in the Vertebrata, on the ventral side of the body, extending along the back of the abdomen. This vessel is often divided into chambers or compartments by valves, having also valvular orifices on its upper side for the flowing in of the fluid (Newport), and giving off vessels (arteries) for its distribution to the rest of the body. In the lower forms, however, of Arachnids (among the Aca-ridea) no such principal muscular vessel is found, the vital fluid being in such cases supposed to circulate generally in the body, and to be distributed irregularly into different portions by the muscular movements of the intestinal canal.

Respiratory System.— Arachnids breathe by means of tracheae (spiracular tubes, as in the Insecta), as well as by pulmo-branchiae, said to be a kind of compound of the gill of fish and the lung of mammals, though in reality there seems better reason, as we shall see further on (p. 293), to consider the pulmo-branchiae as merely peculiarly modified tracheae; both these organs have external orifices (stigmata) variously situated, but commonly in some part of the abdomen. In some Arachnids (certain groups of the Araneidea) respiration is effected by both of the above organs; while in others (certain of the Acaridea) no distinct organs of respiration exist.

Digestive System.— Digestion is effected by a simple alimentary tube or canal running from the mouth to the anus. This tube is various in its form, as well as in the intestines issuing from it; in some groups, as the Araneidea and Acaridea, the above canal has large lateral caecal appendages; in others (Scorpionides and Thelyphonides) the alimentary canal has no such lateral enlargements. In some Arachnids there have also been found a liver (or mass of substance in the abdomen exercising the functions of a liver), an organ performing the part of kidneys, and salivary glands, often of large size, while in others no distinct organs of digestion exist.

Nervous System.— This consists of ganglia or nerve-knots, formed by enlargements of longitudinal nervous cords (generally two in number), differing in position and in the nerves issuing from them. In one large group (Acaridea) where the abdomen usurps, as it were, the place of all the rest of the body, there is no longitudinal nerve cord; a single nerve mass occupies the interior of the abdomen, and sends forth nerves to the various surrounding parts.

Generative System.— Generation is effected by two distinct and separate sexes, the single exception known being the Tardigrada , a group of Acaridea approaching nearly to the Entozoa, and said to be hermaphrodite. In females the parts of generation consist of two sacs (ovaria), one on each side of the alimentary canal. These have a common external orifice (vulva), various in its shape, beneath the anterior extremity of the abdomen; and connected with this opening there is frequently an epigyne, or ovipositor, often of some length, and of characteristic form. In males (except among the phalangids and scorpions) there is no intromittent organ,—the generative parts consisting of two long tubes, similarly situated to the ovaria in females, in which the seminal fluid is secreted; these tubes end in a simple external, very minute orifice, in a similar situation to the vulva of the female.

Arachnids are either oviparous or ovo-viviparous. As before observed, they undergo no proper metamorphosis, but as growth takes place there are more or fewer successive moultings of the skin; and complete development of all the parts does not occur until the last moult, when the creature becomes adult, and so fitted to perpetuate its kind.

With regard to the position held by Arachnids in the great scheme of nature, it seems, from the considerations shortly entered into above, undeniable that they are very closely linked to the rest of the Articulata by much, both of external and internal structure, common to the whole branch. The office they subserve is evidently the keeping down of the superabundant supply of the insect world; some even operating to check the too rapid advance of higher animals.

Assuming that the operation, in some shape or other, of evolution has brought the different groups of Articulata to the varied forms and conditions they now respectively present, it is plain that the higher, at least, of the arachnidous orders must, in all probability, have been modified from some one or more of the other groups, perhaps in the main from the Insecta and Myriapoda,— certainly not the Insecta, for instance, from the Arachnida. Thus, we should define Arachnids, generally, to be insects modified for the purpose of preying upon their congeners; while some of the lower forms of Arachnids, such as Demodex, Tardigrada, and Pentastoma, carry us back to the remote past, before the branch Articulata, in any of its great groups, had attained more than a very slight progress towards its present dignified and highly organised forms.

The above being a general summary of the external and internal organisation of the Arachnida, an examination of the whole group shows us that their different characters, and the ways in which their articulated plan of structure is carried out, are variously correlated and modified, forming structural complications of several marked kinds, whence we get the different orders, being the next subordinate groups, into which Arachnids are divided. These orders are seven in number, and form so many well-defined groups, but of very different extent. Advancing from the more simple to the more highly organised, we begin with the Acaridea, comprising the mites and ticks, and including also the Tardigradides, Pentastomides, as well as Demodex. The second order is that of the Pycnogonidea, marine parasites, which, in a certain plane, appear to connect the Acaridea with the next or third order, Phalangidea; these last are popularly known as “Harvest-men.” Next to them, and forming the fourth order, come the Solpugidea, a small but very distinct group, constituting a link in one plane between the Phalangidea and Scorpionidea, which last, including the Chelifers, or False Scorpions, form the fifth order. Not far removed from these is the sixth order, Thelyphonidea, comprising groups which partake of the characters both of the Scorpionidea and of the next— the seventh and last—order. Araneidea. or true Sniders. [table]

The following abstract shows the chief ordinal characters of these seven groups of Arachnids :—

Order I. Acaridea. —Cephalo-thorax and abdomen united so as to form one piece, generally without mark of union; palpi and falces variable in structure; the several parts of the mouth often united; legs terminating variously, generally eight in number, sometimes only six in the immature state, and in one or two instances but four, even when adult; eyes, when present, variable in number (2 to 6), and placed on the cephalo-thorax; respiration, when proper organs for it exist, wholly tracheal; nervous matter gathered into one large mass, or ganglion; reproduction ovo-viviparous as well as oviparous. Maturity is reached by quasi, or imperfect, metamorphoses in some of the groups.

Order II. Pycnogonidea. —Cephalo-thorax segmented, linear; abdomen rudimentary, forming merely a small terminal segment to the cephalo-thorax; the mouth consisting of the termination of a tubular article forming the head, sometimes accompanied by some more or less distinct parts; legs eight, multi-articulate; in females a supernumerary pair, between the first two and used for bearing ova; organs of respiration, none; metamorphosis imperfect, as in the last order.

Order III. Phalangidea. —Cephalo-thorax and abdomen of tolerably equal size, and united throughout their whole breadth, but the junctional line evidently marked; abdomen annulate or segmentate, caused by transverse folds in the epidermis; falces two-jointed, didactyle; legs in general inordinately long, and very slender; the two terminal joints multi-articulate, and ending with, claws; eyes two, on the vertex of the cephalo-thorax; respiration tracheal; reproduction oviparous; no metamorphosis of even an imperfect kind.

Order IV. Solpugidea. —Cephalo-thorax distinct from the abdomen and segmented, the first segment forming the head; abdomen annulate; falees one-jointed, didactyle; palpi terminating with a capsule containing a peculiar organ, the use of which is not yet known; eyes two, on a small eminence at the fore part of the head in the medial line; respiration tracheal; legs'terminating with two curved fingers; manner of progressive growth unknown.

Order V. Scorpionidea. —Cephalo-thorax formed of one undivided piece, united to the abdomen throughout its entire breadth; abdomen annulate, prolonged (in one of...