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Let us now briefly look at the various appendages carried on these segments, beginning at the head.
The first segment carries à pair of eyes, which are of large size and globular shape, and are supported upon movable stalks. The Lobster can thus roll the eyes about in different directions; and the eyes themselves are what is called “compound," each being composed of numerous simple eyes amalgamated together. The eyes also are protected in part by a great jagged spine or beak developed from the front of the shield which covers the head.
The second ring carries a pair of feelers, which are double, and composed of numerous joints (fig. 12, a). From their small size, these are known as the “lesser antennæ" (Latin, antenna, the yard-arm of a ship).
The third ring carries another pair of feelers (a'), which are known as the “great antennæ,” from their large size. They are composed of numerous joints, like the preceding, but each consists of no more than a single branch.
The fourth, fifth, sixth, and seventh segments carry each a pair of jaws, differing somewhat in each segment, and the last pair so closely approaching the true legs or “feet" in structure as to receive the name of “foot-jaws." All these jaws move from side to side, and are really to be regarded as modified limbs.
The eighth and ninth rings (being the first two rings of the thorax) carry, each, another pair of limb-like jaws or “foot-jaws,” the last pair of these (fig. 12, 12) being of large size and quite like legs. :
The tenth, eleventh, twelfth, thirteenth, and fourteenth segments carry five pairs of legs, which the animal uses partly in walking and partly for grasping. The first pair of these legs is greatly developed (fig. 12, c), and constitutes a pair of great pincers or “nipping-claws.” One claw is blunt, and is used chiefly for holding on to foreign objects, and the other claw is sharply serrated, and is used by the animal for biting or cutting up its food. The second and third pairs of legs (d and e) also terminate in nipping-claws, but these are quite of small size. The fourth and fifth pairs of legs are provided with simply pointed extremities.
The fifteenth segment (being the first of the abdomen) carries in the males) a pair of singular grooved processes(h), and the sixteenth, seventeenth, eighteenth, nineteenth, and twentieth segments carry each a pair of appendages which are known as “swimmerets” (i, j, k, l, m), and each of which consists of an undivided base, terminated by two flattened paddles or oars (fig. 13, 2). The last of these pairs of swimmerets (m) has the terminal paddles greatly expanded.
The twenty-first segment (t) carries no appendages, and · is simply placed between the expanded swimmerets of the
twentieth ring, thus constituting a most powerful tail-fin, by the strokes of which the animal can propel itself through the water, tail foremost, for an astonishing distance, and with great rapidity.
The following table shows the segments of which the Lobster is composed, with their proper appendages :
3d, Greater antennæ.
5th, First pair of chewing-jaws.
loth, First pair of legs (great claws). THORAX, . rith, Second pair of legs (small claws).
12th, Third pair of legs (small claws).
20th, Large swimmerets.
| 21st, No appendages. We may add to this description of the Lobster, that not only is the body composed of a succession of segments, some of which are movably jointed to one another, but the limbs are also composed of distinct pieces or joints, and are movably jointed or “articulated” to the body. Hence the Lobster is emphatically what is called an “ Articulate” animal (Latin, articulatus, jointed).
It remains very briefly to consider the more important points in the internal anatomy of the Lobster. The mouth is placed on the under surface of the head, and in addition to the numerous jaws already alluded to, it is further provided with an upper and lower lip, both of a shelly nature. The mouth leads by a gullet to a globular stomach, from which an intestine proceeds, to terminate by a distinct vent at the base of the last segment of the body; and there is a well-developed liver.
The heart (fig. 14, h) is placed upon the back, and drives the pure blood, which has passed through the gills, to all parts of the body. The breathing-organs (66) are adapted for breathing air dissolved in water, and are therefore genuine “gills.” They are in the form of pyramidal bodies, which are attached to the bases of the
Fig. 14.-Diagram to show the position of the internal organs of the Lobster
as they would be seen if the animal were cut across behind the head. h Heart; bb Gills (the vessels containing pure or arterial blood are le light, those containing venous or impure blood are dark); i Intestine : n Nervous system.
legs, and are concealed from view beneath the sides of the great shield which covers the head and thorax. Owing
to their being attached to the legs, the animal's respiratory process depends very much upon its moving about, since the movements of the legs contribute considerably to the bringing in of fresh water to the chambers in which the gills are contained.
The nervous system, lastly, has the form of a series of nervous masses placed along the lower surface of the body, and united with one another by longitudinal cords. The first pair of these masses is placed above the gullety and the cords which unite them with the next pair pass on each side of the gullet, so that the gullet is surrounded by a "nerve-collar.”
The Lobsters are exclusively found in the sea; and though they can live a considerable time out of the water, they are essentially aquatic animals. They are exceedingly voracious, and are usually captured by means of “lobster-pots,” or baskets baited with some kind of carrion or garbage. When injured, or even if greatly alarmed, they throw off one or both of the great claws; but these appendages soon grow again, though not so large as before. They also cast their shells periodically, since the resisting nature of this covering does not allow of their growth. When fresh they are very brightly coloured; but they turn to a uniform and brilliant red when boiled. They are most ordinarily about a pound in weight, but they sometimes grow to three or four pounds.
RECAPITULATION OF ESSENTIAL CHARACTERS.—The skin is hardened with lime and horny matter, so as to form a resisting shell or “crust," within which the internal organs are protected. The body consists of a succession of distinct rings or segments placed one behind the other; and each segment may carry a single pair of jointed appendages. The animal breathes air dissolved in water, and usually has breathing-organs in the form of gills. The nervous system consists of a chain of nervous masses placed along the lower surface of the body. Less essential, though highly distinctive, are the characters that the true legs are from five to seven pairs in number, that the segments of the abdomen carry appendages, that there are
two pairs of antennæ, and that there is a distinct heart placed upon the back. By these characters the class of the Crustacea is distinguished as a whole.
THE class Arachnida (Greek, arachne, a spider) comprises the Scorpions, Spiders, Mites, and Ticks. As an excellent example of this class we may select the common House Spider (Tegenaria civilis) of Britain. The body of this familiar animal (fig. 15, A) in reality resembles that of the Lobster, in being composed of a series of rings or segments, placed one behind the other; but these segments are not conspicuous, and the body only shows a well-marked division into two distinct portions—a front portion carrying the legs, and a hinder portion carrying no appendages.
The skin over the whole body is more or less hardened with horny matter; but more so in some parts than in others, and it nowhere forms a shell like that of the Lobster.
The front portion or half of the body (fig. 15, A, c) is in reality composed of the head and trunk (“thorax"), so consolidated together that no sign of a boundary between them can be made out, and that no distinct segments can be detected. On the sides of this region of the body we observe four pairs of long, jointed legs, and in front of these a pair of what look like small legs. These latter organs (p), however, are not really legs, but are a sort of feelers which are attached to the jaws. The spider, therefore, has truly four pairs of legs, and it should thus never be confounded with the genuine Insects, all of which possess no more than three pairs of legs. All the legs are long and slender, covered with numerous short hairs and a few longer spines, terminated by three claws each, and