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composed of seven distinct joints or pieces. They are fixed at their bases to a strong horny plate which covers the chest in front.
Fig. 15.-A, The male of the common House Spider (Tegenaria civilis), con
siderably magnified ; c Front portion of the body, consisting of the amalgamated head and thorax; p Feelers attached to the jaws; a Abdomen. B, Front portion of the head of the same, showing the eight eyes (J), and the poison-jaws (n). C, Under side of the head and trunk, showing the true jaws (m), the lower lip (1), 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.)
Upon the front of the head are situated eight simple eyes (fig. 15, B, f), arranged in two lines, the pairs which occupy the ends of the line being placed on small tubercles. On the under surface of the head is situated the opening of the mouth, and in front of this are two organs,
which are often spoken of as jaws, but which really correspond with the feelers ("antennæ") of insects. These singular organs (fig. 15, B, n) are in the form of two powerful jaw-like structures which terminate in strong curved hooks, and are movably jointed to the head. The hooks are perforated by a minute aperture at the point, communicating with a poison-gland ; so that when one of these fangs is struck into the body of an insect, a drop of a poisonous fluid is forced out into the wound. In this way the spider kills the small animals upon which it feeds.
The mouth is closed behind by a plate representing the lower lip (fig. 15, C, l), and has at its sides two genuine jaws (m), which carry the jointed feelers already alluded to.
The front portion of the body of the Spider, as before remarked, is truly the amalgamated head and thorax. Behind this, and united with it by a narrow stalk, is the egg-shaped, hairy mass, which constitutes the hinder half of the body (fig. 15, A, a). This, though not exhibiting any distinct segments, is really the “abdomen," and corresponds with the "tail" of the Lobster. It does not support any legs or appendages, but at its hinder end are situated three pairs of minute conical eminences, which spin the fibres which compose the web, and which are termed the 66 spinnerets." The substance which composes the fibres out of which the web is constructed, is secreted within the body in a fluid form by certain special glands. The fluid silk is then cast into its proper thread-like form by being passed through the spinnerets, which are perforated by numerous very minute pores or holes. In this way the silken thread of the web is really composed of numerous very delicate filaments woven together.
Upon the under surface of the abdomen, far forwards, are situated two small openings which communicate with the breathing-organs. These latter have the form of little sacs or air-chambers, the lining membrane of which is thrown into numerous folds like the leaves of a book (fig. 15, D), and is richly supplied with blood. The air is admitted directly to these sacs, and the blood is thus purified ; so that the Spider is an air-breathing animal.
As regards the internal anatomy of the Spider, little need be said. The digestive system presents no remarkable peculiarity, except that the throat is extremely narrow; a wide throat not being necessary for an animal which lives merely upon the juices of its prey. The intestine terminates in a distinct vent, and there is a well-developed liver. The heart has the form of a long tube placed along the back; and the nervous system has, in the young, the form of a chain of nervous masses placed along the lower surface of the body. In the adult, however, these masses are aggregated and amalgamated with one another.
Tegenaria civilis attains a length of nearly half an inch, and is the common House Spider of Britain, It spins a horizontal web, appended to which is a short open tube, into which the animal retreats when threatened with danger, and from which it watches for insects which may fall into its snare. It is mostly of a reddish-brown colour, with black markings. It lays its eggs in little packets of fifty or sixty each, all kept together by delicate silken fibres. Several of these packets are constructed and are attached to walls or other objects in the neighbourhood of the web. The young Spider is like its parent, but is much smaller; and it changes its skin no less than nine times before it assumes the characters of the adult. Its entire span of life appears to extend over four years.
RECAPITULATION OF ESSENTIAL CHARACTERS. — The body is composed of a series of rings placed one behind the other; and those rings which belong to the head and trunk (thorax) are amalgamated together. There are four pairs of legs. The abdomen never carries legs. The animal breathes air directly. The heart (if present) is situated on the back, and the nervous system has, at any rate to begin with, the form of a chain of little nervous masses placed along the lower surface of the body. The feelers (“antennæ") which are so characteristic of Insects, when present at all, are converted into offensive weapons (poison-fangs or pincers). These characters distinguish the Arachnida as a whole.
THE Myriapods are commonly known as Centipedes and Millipedes, and all these names refer to the great number of the feet or legs, as compared with Insects or Spiders, (Greek, murios, ten thousand ; podes, feet). As a good representative of this class may be taken Lithobius forficatus, one of the commonest of British Centipedes.
The body in Lithobius (fig. 16, A) is elongated and flattened, and exhibits very distinctly a division into two regions-namely, a head (h), and a lengthy body composed of a series of separate rings or segments, each of which carries a single pair of appendages. The entire integument, both over the head and over the body, is hardened, so as to form a strong case, within which the internal organs are contained; and the appendages to the segments are very distinctly jointed.
The head is somewhat heart - shaped, and though it really consists of several pieces or segments, these are so consolidated that it appears to form a single piece. The head carries a single pair of long, jointed feelers, known as the "antennæ" (Latin, antenna, the yard-arm of a ship), which consist of very numerous short joints. The head also carries upon its two sides a collection of minute simple eyes (fig. 16, C, e), from twenty-two to twenty-four on each side. On its under surface the head bears the opening of the mouth, with a well-marked lower lip behind it—this latter organ being really double (fig. 16, B, 1).
Immediately behind the head, and looking as if it belonged to it, is a narrow ring which carries a pair of powerful jaws (fig. 16, A, F, and B, f). These jaws are strongly hooked, and are perforated for the purpose of conveying a poison, with which the Centipedes kill their prey, or defend themselves against their enemies. Though
officiating as jaws, these hooked fangs are really to be regarded as being modified legs or “feet,” and they are, therefore, called "foot-jaws."
Fig. 16.-A, Lithobius forficatus, enlarged, and viewed from above; an An
tennæ; f Foot-jaws; h Head. B, Head of Lithobius Leachii, viewed from below (after Newport); an Antennæ; f Hooked foot-jaws; I Lower lip, composed of two pieces. C, Head of Lithobius forficatus, viewed from above (after Gervais); an Antennæ; e Eye.
Behind the foot-jaws, the body exhibits fifteen distinct rings, alternately large and small (fig. 16, A), and each of these rings carries a pair of jointed legs. There are therefore fifteen pairs of legs, of which the hindmost are longer than the others, and are directed backwards in the line of the body, so as to form a kind of tail.