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moving objects at a little distance and then close to the animal. You may also test to see if the animal can distinguish light from darkness. This may be done by covering half of the tray in which the crayfish is confined. Then place the animal in the light end of the tray to see if it will travel toward the dark end of the tray. This may be repeated by placing the animal in the dark end. Thus it will be possible to discover the reaction of the animal to the light. Notice the position of the stalked eyes. the eye with a pencil; is it freely movable? In what direction? how well the eye is protected from injury. The anterior end of the carapace projects to form a spiny process; this, with the socket in which the eye rests and its position on the side of the head, forms ample protection
Mouth parts of the crayfish; 1, walking appendage, showing attachment of gill; 2, the jaw, with palp; 3, first maxilla (second maxilla not shown); 4, third maxilliped; 5, second maxilliped, showing baler; 6, first maxilliped, showing gill attached; 7, swimmeret; 8, uropod.
to this important organ. The eyes of the crayfish, like those of an insect, are compound. They differ from those of the insect in being borne on stalks. If a small bit of the exoskeleton covering the eye is placed under the compound microscope, it will be found to be made up of a number of little rectangles; this shows the size and shape of a surface view of the units composing the compound eye.
If it is possible to have the aquarium holding the crayfish in the schoolroom, the method of feeding may be watched. Notice that the pincher claws (chelipeds) are used to hold and tear food, as well as for defense and offense. Living food is obtained with the aid of the chelipeds. Food is shoved by the chelipeds toward the mouth; it is assisted there by several small appendages called foot jaws (maxillipeds) and to a slight degree by two still smaller paired maxillæ just under the maxillipeds. Ultimately the food reaches the hard jaws and, after being ground between them, is passed down to the stomach. If you hold the crayfish in such a position that you can pour a little beef juice or other edible fluid over the mouth parts, it will be possible to observe the mouth parts work as they do in a state of nature.
The mouth parts of a crayfish resting in the aquarium are observed to be constantly in motion, despite the fact that no food is present. If the crayfish is taken out of the water and held with the ventral surface upmost, a little carmine (dissolved in water) may be dropped on the lower surface of the animal. This carmine runs down under the carapace. If now the animal is held in water in the same position, the carmine will appear from both sides of the mouth, seemingly propelled by something which causes it to emerge in little puffs. If we remove the maxillipeds and maxillæ from a dead specimen, we find a groove leading back from each side of the mouth to a cavity of considerable size on each side of the body under the carapace. This is the gill chamber. It contains the gills, the organs which take oxygen out of the water. The second maxillæ are prolonged down into the groove to serve as bailers or scoops. By rapid action of this organ a current of water is maintained which passes over the gills.
The gills are outside of the body, although protected by the carapace. If the carapace is partly removed on one side, they will be found, looking somewhat like white feathers. The blood of the crayfish passes by a series of vessels into the long axis of the gill; in this organ the blood tubes divide into very minute tubes, the walls of which are extremely delicate. Oxygen, dissolved in the water, passes into the blood by osmosis, during which process the blood loses some carbon dioxide. Notice that the gills are kept from drying by being placed in a nearly closed chamber, which is further adapted to its function by means of the row of tiny hairs which border the lower edge of the carapace.
Crayfish with the left half of the body structures removed; a, intestine; b. ventral artery; c, brain; e, heart; et, gastric teeth; i, oviduct; 7, digestive gland; m, muscles; n, green gland (kidney); o, ovary; p, pyloric stomach; r, nerve cords; 8, cardiac stomach; st, mouth; u, telson; w, openings of veins into the pericardial sinus. Twice natural size. Davison, Zoology.
The laboratory exercise should conclude with a drawing of the animal from the side, about natural size, with part of the carapace cut away to show the gills. Show as many of the above-mentioned parts as possible. For other useful drawings see Hunter and Valentine, Manual, page 124.
CIRCULATION. - The circulation of blood in the crayfish takes place in a system of thin-walled, flabby vessels which are open in places, allowing the blood to come in direct contact with the tissues to which it flows. The heart lies on the dorsal side of the body, inclosed in a delicate bag, into which all the blood in the body eventually finds its way during its circulation.
DIGESTION. Food which is not ground up into pieces small enough for the purpose of digestion is still further masticated by means of three teeth, strong projections, one placed on the midline and two on the side walls of the stomach. The exoskeleton of the crayfish extends down into the stomach, thus forming the gastric mill just described.
The stomach is divided into anterior and posterior parts separated from each other by a constriction. The posterior part is lined with tiny projections from the wall which make it act as a strainer for the food passing through. Thus the unbroken particles of food are kept in the anterior end of the stomach. Opening into the posterior end of the stomach are two large digestive glands which further prepare the food for absorption through the walls of the intestine. Once in the blood, the fluid food is circulated through the body to the tissues which need it.
NERVOUS SYSTEM. — The internal nervous system of a crayfish consists of a series of collections of nerve cells (ganglia) connected by means of a double line of nerves. Posterior to the gullet this chain of ganglia is found on the ventral side of the body, near the body wall. It then encircles the gullet and forms a brain in the head region, the latter formed from several ganglia which have grown together. From each of the ganglia, nerves pass off to the sense organs and into the muscles of the body. These nerve fibers are of two sorts, those bearing messages from the outside of the body to the central nervous system (these messages result in sensations), and those which take outgoing messages from the central nervous system (motor impulses), which result in muscular movements.
DEVELOPMENT.-The sexes in the crayfish are distinct. The developing eggs, which are provided with a considerable supply of food material, are glued fast to the swimmerets of the mother, and there develop in safety. The young, when they first hatch, remain clinging to the swimmerets for several weeks.
EXCRETION OF WASTES. - On the basal joint of the antennæ are found two projections, in the center of which are found tiny holes. These are the openings of the green glands, organs which have the function of the elimination of nitrogenous waste from the blood, the function of the human kidneys.
Characters of Crayfish and its Allies. Our study of crayfish shows us that animals belonging to the same group as itself have several well-marked characteristics. The most important are the presence of a segmented limy exoskeleton, gills, appendages, usually a pair to each segment of the body (except the last); and, as we shall see later, they pass through a metamorphosis or change of form before they reach the adult state. We find that the Crustacea fall naturally into two classes, those in which the number of pairs of appendages is indefinite, and those in which the number is fixed
at nineteen. In this latter class are placed the crayfish, lobster, blue crab, shrimp, and most of our common crustaceans.
The North American Lobster. - In structure it is almost the counterpart of its smaller cousin, the crayfish. Its geographical range is a strip of ocean bottom along our coast, estimated to vary from thirty to fifty miles in width. This strip extends from Labrador on the north to Delaware on the south. The lobster is highly sensitive to changes in temperature. It migrates from deep to shallow water or vice versa according to the temperature of the water, which in winter is relatively warmer in deep water and cooler in shallows. Sudden changes in the water of a given locality may cause them to disappear from that place. The more abundant food supply near the shore also aids in determining the habitat of the lobster. Lobsters do not appear to migrate north and south along the coast. While little is known about their habits on the ocean bottom, it is thought that they construct burrows somewhat like the crayfish, in which they pass part of the time. As they have the color of the bottom and as they pass much of their time among the weedcovered rocks, they are able to catch much living food, even active fishes falling prey to their formidable pinchers. They move around freely at night, usually remaining quiet during the day, especially when in shallow water. They eat some dead food; and thus, like the crayfish, they are scavengers.
Development. The female lobsters begin to lay eggs when about seven inches in length. Lobsters of this size lay in the neighborhood of five thousand eggs; this number is increased to about ten thousand in lobsters of moderate size (ten inches in
length); in exceptionally large specimens as many as one hundred thousand eggs are sometimes laid. The eggs are laid every alternate year, usually during the months of July and August. Eggs laid in July or August, as shown by observations made along the coast of Massachusetts, hatch the following May or June. The eggs are provided with a large supply of yolk (food), the development of the young animal taking place at the expense of this 1ood material. After the young escape from the egg they are aln ost transparent and little like the adult in form. During this per od of their lives the mortality is very
great, as they are the prey of many fish and other free-swimming animals. It is estimated that barely one in five thousand survives this period of peril. At this time they grow rapidly, and in consequence are obliged to shed their exoskeleton (molt) frequently. During the first six weeks of life, when they swim freely at the surface of the water, they molt from five to six times.1
Metamorphosis of a shrimp; a, nauplius or earliest stage; b, c, d, later larval stages; e, adult. Note that as the animal grows more appendages appear, and that these develop backward from the anterior end.
Molting. During the first year of its life the lobster molts from fourteen to seventeen times. During this period it attains a length of from two to three inches. Molting is accomplished in the following manner: The carapace is raised up from the posterior side and the body then withdrawn through the opening between it and the abdomen. The most wonderful part of the process is the withdrawal of the flesh of the large claws through the very small openings which connect the limbs with the body. The blood is first withdrawn from the appendage; this leaves the flesh in a flabby condition (a
1 Recent economic investigations upon the care of the young developing lobster show that animals protected during the first few months of free existence have a far better chance of becoming adults than those left to grow up without protection. Later in life they sink to the bottom, and because of their protectively colored shell and the habit of hiding under rocks and in burrows, they are comparatively safe from the attack of enemies.