mussels, oysters, and clams, possessing certain characters in common, together form a natural group of animals.

There are, however, many other animals which are not insects, spiders, or myriapods, and still possess a body composed of segments, and also have jointed legs, and these animals are to furnish the subject for the next lesson.

CHAPTER XVII.

CRAWFISH AND LOBSTER.

120. THE fresh-water Crawfish, or fresh-water Lobster as it is sometimes called, is very common in many of the Western rivers. It may be collected in little pools by the riverside, and kept alive for a long while in a jar of water. It may be fed on fresh-water snails and the larvæ of insects. It would be well to keep the animal alive for a while, so that its motions in swimming and crawling may be observed. For the cabinet, it can be dried with the legs outstretched, or specimens may be preserved in alcohol.

The general form of the body is much like that of the salt-water lobster, differing, however, greatly in size; the crawfish varying from three to five inches in length, and the lobster attaining a much larger size.

The animal is divided into two regions, the body proper, to which the legs and big claws are attached, and the abdomen, consisting of the jointed portion behind. The head

does not appear separated from the body as in the insects, but is combined with the thorax, and hence this part is called the cephalo-thorax, as in the spiders. The cephalo-thorax is covered by a continuous shield, or shell, called the carapace, while the abdomen is divided into a series of segments. This part can be bent snugly beneath the body (see Fig. 121).

FIG. 121.-FRESH-WATER CRAWFISH FROM THE MISSISSIPPI RIVER.

At the hinder end of the abdomen are five flattened appendages, which serve as fins, by means of which the animal can swim vigorously backward. (See Fig. 122.)

FIG. 122.-TAIL OF CRAWFISH SHOWING FLATTENED APPENDAGES FOR SWIMMING.

There are two compound eyes in front, which rest upon little jointed stalks, so that the creature can turn them in various directions.

121. Just below the eyes are two long, and two short and double antennæ, or feelers, and directly below these are six pairs of variously shaped and jointed appendages closely packed together. They surround the mouth and assist in securing and preparing the food for the stomach. The first pair are called jaws, or mandibles, and are furnished with sharp cutting edges for biting the food, and a flattened ́surface for grinding or crushing it. The next two pairs are called maxillæ, and are accessory jaws. The pair of mandibles and the two pairs of maxillæ, with another pair just behind, making four pairs in all, belong to the head, the other two pairs of mouth-parts belong to the thorax, and are so evidently modified claws or feet that they, with the pair just in front of them, are called foot-jaws or maxillipedes. (See Fig. 126, in which these parts are all named.)

122. From the under side of the body project five pairs of jointed legs, and these differ in shape and size. The first pair are much larger than the rest, and in the lobster are

a

FIG. 123.-A BIG CLAW OF THE LOBSTER, SHOWING THE WOODEN WEDGE, A.

called the big claws. They carry at their extremities big pincer-like jaws capable of giving a sharp nip, and these are used as weapons of defence, and also to hold on to their prey. The lobster can bite very severely with these big

claws, and for this reason the fisherman drives in a little wedge of wood to prevent the animal from opening the movable part, so that he can handle it without being bitten.

The other legs are long and slender. The two forward pairs end in slender nippers, while the two hinder pairs end in a single projecting claw. With these four smaller pairs of legs the crawfish and lobster crawl or walk.

On the under side of the abdomen are little flattened appendages arranged in pairs, a pair to each ring or segment. The animal not only swims backward by means of the broad

FIG. 124.-ONE OF THE FLATTENED OR ABDOMINAL APPENDAGES OF A LOBSTER.

fins on the end of the tail, or abdomen, but has the power besides to swim in a forward direction by extending the abdomen, and using the little fins below as swimming organs.

123. The crawfish, like the lobster, breathes in the water by means of gills. These are attached to the base of the legs and are concealed on the sides of the thorax by the carapace, which covers them. By forcibly tearing up the side of the carapace, there will be exposed the gills which

look like plumes. This space may be called the gill-chamber,
and the water flows into it by passing under the edge of the
carapace back of the big claws, and passes out of an opening
near the mouth-parts. The currents of water flowing in to
the gill-chamber are induced by a stiff appendage attached to
the base of the second pair of maxillæ called the flabellum
(see Fig. 125), and which swings back and forth and scoops
the water into this chamber. These gills are shown as they
appear in the crawfish.
In tearing off the claws of the

lobster, the gills are often drawn out too, and remain attached
to the base of the legs. In the lobster the carapace can be
easily bent up, so as to show the gills.

In the following figure a crawfish is shown with a portion

FIG. 125.-CRAWFISH SEEN FROM THE SIDE, WITH THAT PORTION OF THE CARAPACE REMOVED
WHICH COVERS THE BRANCHIE OR GILLS. THE APPENDAGES OF THE LEFT SIDE ONLY
SHOWN.-8, Region of Stomach; A, Abdominal Appendages; B, Bases of the Four Small
Legs; C, Base of Large Claw; f, Flabellum attached to the Second Maxillipede; e, Eye.