The Arachnida - including the Spiders, Scorpions, Mites, etc. - possess almost all the essential characters of the Crustacea, to which they are very closely allied. Thus, the body is divided into a variable number of somites, some of which are always provided with articulated appendages. A pair of ganglia is primitively developed in each somite, and the neural system is placed ventrally. The heart, when present, is always situated on the opposite side of the alimentary canal to the chain of ganglia. The respiratory organs, however, whenever these are differentiated, are never in the form of branchiae as in the Crustacea, but are in the form either of pulmonary vesicles or sacs, or of ramified tubes, formed by an involution of the integument, and fitted for breathing air directly. Further, there are never "more than four pairs of locomotive limbs, and the somites of the abdomen, even when these are well developed, are never provided with limbs;" the reverse being the case amongst the Crustacea. Lastly, "in the higher Arachnida, as in the higher Crustacea, the body is composed of twenty somites, six of which are allotted to the head; but in the former class, one of the two normal pairs of antennae is never developed, and the eyes are always sessile; while, in the higher Crustacea, the eyes are mounted upon movable peduncles, and both pairs of antennae are developed " (Huxley).
The head of the Arachnida is always amalgamated with the thorax, to form a "cephalothorax;" the integument is usually chitinous, and the locomotive limbs are mostly similar in form to those of insects, and are usually terminated by two hooks.
In many of the Arachnida the integument remains soft over the entire body; in others, as in the majority of Spiders, the abdomen remains soft and flexible, whilst the cephalothorax is more or less hard and chitinous; in the Scorpions, again, the integument over the whole body forms a strong chitinous shell. The cephalothorax may be segmented (Solpugidae); and the abdomen may or may not be segmented. Though four pairs of legs are present, the first is certainly homologous with the labial palpi of the Insecta.
The typical somite of the Arachnida is constituted upon exactly the same plan as that of the Crustacea, consisting essentially of a dorsal and ventral arc; the former composed of a central piece, or "tergum," and of two lateral pieces, or "epimera;" whilst the latter is made up of a median "sternum " and of two lateral "episterna."
As regards the composition of the cephalothorax of Spiders, "the tergal elements of the coalesced segments are wanting, and the back of the thorax is protected by the elongation, convergence, and central confluence of the epimeral pieces; the sternal elements have coalesced into the broad plate in the centre of the origins of the ambulatory legs, from which it is separated by the episternal elements. . . . The non-development of the tergal elements explains the absence of wings" (Owen).
The mouth is situated, in all the Arachnida, in the anterior segment of the body, and is surrounded by suctorial or masticatory appendages. In the higher Arachnida, the mouth is provided from before backwards with the following appendages (figs. 161, 162): 1. A pair of "falces," or "mandibles," used for prehension; 2. A pair of "maxillae," each of which is provided with a long jointed appendage, the "maxillary palp;" 3. A lower lip, or "labium." In the Scorpion, an upper lip, or "labrum," is also present.
Fig. 161. - A, The male of the common House-Spider (Tegenaria civilis), considerably magnified : c Front portion of the body, consisting of the amalgamated head and thorax ; p Maxillary palpi; a Abdomen. B, Front portion of the head of the same, showing the eight eyes (f), and the mandibles (n). C, Under side of the head and trunk, showing the true jaws (m), the lower lip (/), and the horny plate to which the legs are attached. D, Diagram of one of the air-chambers or breathing-organs. (Figs. A, B, and C are after Blackwall.)
* The nomenclature ordinarily applied to the parts of the mouth in the Arachnida is a misleading one, so far as the homologies of this class with the Insecta are concerned. Thus the so-called "mandibles" are really the antennae ; the "mandibles" themselves are absent, but the "chelae" of the Scorpions may really represent the " mandibular palpi;" whilst the first pair of legs really correspond with the il labial palpi," and the second pair of legs may possibly be a modification of a second pair of palps.
In the Spiders (fig. 162, 4) each falx or mandible terminates in a sharp movable hook, which possesses an aperture at its extremity communicating by a canal with a gland, which is placed in the preceding joint of the mandible, and secretes a poisonous fluid. The maxillary palps in the Spiders are long, jointed appendages, terminated in the females by pointed claws, but frequently swollen, and carrying a special sexual apparatus in the males.
In the Scorpions (fig. 162, 1) the mandibles are short and terminate in strong pincers, or "chelicerae." The maxillary palpi are also greatly developed, and constitute powerful grasping claws, or " chelae." In the genus Galeodes, the mandibles, like those of the Scorpion, constitute "chelicerae," though comparatively much larger and longer; but the maxillary palps are not developed into "chelae."
Fig. 162. - Morphology of Arachnida, 1. Organs of the mouth in the Scorpion, on one side: m Mandibles (antennae) converted into chelae, and called the chelicerae; p Maxillary palpi greatly developed, and forming strong chelae. 2. Telson of the Scorpion. 3. One of the abdominal segments of the Scorpion, showing the "stigmata," or apertures of the pulmonary sacs. 4. Tegenaria chvilis, the common Spider (male), viewed from below: s Spinnerets; m Mandibles with their perforated hooks - below the mandibles are the maxillae, and between the bases of these is the , labium ; p The maxillary palpi with their enlarged tumid extremities.
With regard to antennae, these organs, as such, do not exist in the Arachnida. It is generally believed, however, that the mandibles of the Arachnida are truly homologues, not of the parts which bear the same name in the other Arthropoda, but of the antenna; and the name of "falces" is thus best applied to them. The antennae, therefore, of the Spiders are converted into prehensile and offensive weapons; whilst in the Scorpions, as in the King-crabs, they are developed into nipping-claws, or chelae.
In the lower Arachnida, the organs of the mouth, though essentially the same as in the higher forms, are often enveloped in a sheath, formed by the labium and maxillae, whilst the mandibles are often joined together so as to constitute a species of lancet.
The mouth conducts by an oesophagus, sometimes by the intervention of a pharynx, to the stomach, which often carries longer or shorter caeca appended to it. The intestinal canal is short and straight, no convolutions intervening between the mouth and the anus. The terminal portion of the intestine is generally dilated into a cloaca, into which open, as a rule, branched or tortuous tubes, supposed to have a renal function, and to correspond with the "Maipighian vessels" of Insects. Salivary glands are generally present, and there is usually a well-developed liver.
The circulation in the Arachnida is maintained by a dorsal heart, which is situated above the alimentary canal, and is wanting in the lower forms. Usually the heart is greatly elongated, and resembles the "dorsal vessel" of the Insecta. In the lower Arachnida, however, there is no central organ of the circulation, and there are no differentiated blood-vessels. All the Arachnida, except some of the lowest, breathe the air directly, and the respiratory function is performed by the general surface of the body (as in the lowest members of the class), or by ramified air-tubes, termed "tracheae," or by distinct pulmonary chambers or sacs; or, lastly, by a combination of tracheae and pulmonary vesicles. The "tracheae" consist of ramified or fasciculated tubes, opening upon the surface of the body by distinct apertures, called "stigmata." The walls of the tube are generally prevented from collapsing by means of a chitinous fibre or filament, which is coiled up into a spiral, and is situated beneath their epithelial lining. The pulmonary sacs, or "tracheal lungs," are simply involutions of the integument, abundantly supplied with blood; the vascular surface thus formed being increased in area by the development of a number of close-set membranous lamellae, or vascular plates, which project into the interior of the cavity. Like the tracheae, the pulmonary sacs communicate with the exterior by minute apertures, or "stigmata" (fig. 162, 3), and they are to be regarded as being simply greatly expanded tracheae.
The nervous system is of the normal articulate type, but is often much concentrated. Typically there is a cephalic or "cerebral" ganglion, a large thoracic ganglion, and often small abdominal ganglia. In some of the lower forms the articulate type of nervous system is lost, and there is merely a ganglionic mass situated in the abdomen. In none of the Arachnida are compound eyes present, and in none are the eyes supported upon foot-stalks. The organs of vision, when present, are in the form of from two to eight or more simple eyes, or "ocelli." In all the Arachnida, with the exception of the Tardigrada, the sexes are distinct. The great majority of the Arachnida are oviparous, and in most cases the larvae are like the adult in all except in size. In some cases, however (Acarina), the larvae have only six legs, and do not attain the proper four pairs of legs until after some moults.