This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(935). The Arachnidans, long confounded with Insects, and described as such even by recent entomologists, are distinguished by characters of so much importance from the animals described in the last chapter, that the necessity of considering them as a distinct class is now no longer a matter of speculation. In Insects, the external skeleton presents three principal divisions - the head, the thorax, and the abdomen: but in the Spider tribes, the bloodthirsty destroyers of the insect-world, the separation of the head from the thorax, which by increasing the flexibility, necessarily diminishes the strength of the skeleton, is no longer admissible; and, the process of concentration being carried a step further, the head and thorax coalesce, leaving only two divisions of the body recognizable externally, viz. the cephalo-ihorax and the abdomen. Insects, in their mature forms, were found to be invariably furnished with only six legs, but in the adult Arachnidans eight of these limbs are developed. These characters in themselves would be sufficient to discriminate between the two orders; but when to these we add that in the Arachnidans the eyes are invariably smooth, that the antennae of Insects are represented by organs of a totally different description, that the sexual apertures are either situated beneath the thorax, or at the base of the abdomen, and, moreover, that in the greater number of Arachnidans respiration is carried on in localized lungs (jpulmonibranchice), instead of by tracheae as in Insects, we need not enlarge further in the present place upon the propriety of ranking the Arachnida as a separate class.
These animals may be grouped under three principal divisions, - the first of which is evidently an intermediate type of organization, combining many of the characters of the Insecta with the external limbs and palpi of proper Arachnida.
* Verhandl. d. Wurzb. Phys. Med. Ges. viii. 1857.
(936). The Arachnida Trachearia, in fact, breathe by means of tracheae resembling those of Insects, which are so arranged as to convey air to every part of the system; and we may therefore suppose that their circulatory apparatus, as well as their secerning organs, conform more or less to the type of structure met with in the class last described. The Mites (Acaridce) belong to this division, and form a very numerous family, which is extensively distributed. Some are parasitic in their habits, infesting the bodies of insects; and one, the itch-insect (Acarus scabiei), is found occasionally upon the human skin. Many live in cheese and other provisions, where they multiply prodigiously; and not a few inhabit leaves, or are found under stones or beneath the bark of trees. Some (Hydrachna) are aquatic; but, unfortunately, in all, from their extremely minute size, the investigation of their internal viscera presents so many difficulties, that but little is satisfactorily known concerning their anatomy: even the pseudo-Scorpioniclce, which are of larger growth and, although still breathing by tracheae, approximate most closely to the outward form of the next group, have been very imperfectly examined.
(937). In the Acaridans, the most remarkable feature of their structure is the complete consolidation or coalescence of the principal divisions of the body, which are always more or less distinct in the other Articulata; for not only do we find in them the head consolidated with the thoracic portion, but the abdomen likewise is swallowed up, as it were, in the general covering of the body. The legs, as in other Arachnidans, are eight in number, and are generally composed of seven articulations, of which the first, which is sometimes adherent and sometimes free, corresponds with the coxa of Insects, the second with the trochanter; the third, representing the femur, is often more developed than the rest, whilst the remaining constitute the tibia and the tarsal joints. The last segment of the tarsus, or foot as it might be called, is furnished with two moveable hooks, that can be folded back, and lodged in a slight excavation provided for the purpose.
(938). In accordance with their structure, which is adapted to the habits of the various races, the feet of the Acaridans may be divided into such as are adapted for feeling (palpatorii), in which the ultimate joint is dilated; for walking (gressorii); for swimming (remigantes), having the last joint expanded and ciliated, as in some, but not all, of the aquatic tribes; for running (cursorii), where it is long and slender; for weaving (textorii), in which case the ultimate segment is provided with very short and much-curved hooks, and the antepenultimate with four elongated stiff bristles longer than the foot; and lastly, such as are formed for a parasitic life, or carunculated (carunculati), in which, superadded to the hooks, is a caruncle or broad membrane wherewith the creature fixes itself to a smooth surface, something in the same way as the sucker of a Leech.
(939). The mouth is composed of two moveable pieces called the mandibles, beneath which is a broad plate (labium), which is either flat or folded laterally so as to form a kind of gutter, and, moreover, is furnished on each side with a rudimentary palpus. The mandibles are generally free, but in some cases are united together and conjoined with the labial piece, so as to form a short tube or proboscis, near the end of which may be perceived a pair of moveable tooth-like structures, adapted to pierce the substances whence these suctorial races obtain their liquid food. When the mandibles remain entirely free and moveable, they exhibit, as was pointed out by M. Duges*, three principal modifications in their structure: 1st, they are forcipated (fig. 184), like those of Scorpions; secondly, they may be terminated by a single moveable fang (fig. 186, c), as is the case in Spiders; and lastly, they may be composed of two long styles which are capable of alternate movements backwards and forwards, whereby they can perforate foreign substances, much in the same manner as the saw of the Tenthredo among insects. The first of these forms are never provided with any poison-apparatus, and are only adapted to tear and pull to pieces alimentary substances; but in the second form poison-glands are superadded to the curved fangs, which, as in the proper Arachnidans, thus become formidable weapons.