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 ergs are provided with a large supply of yolk (food), the-development of the young animal taking place at the expense of this food material. After the young escape from the egg they are almost transparent and little like the adult in form. During this period 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 back ward 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.

state similar to the taproot which has lost water by osmosis) so that the muscles can be drawn through without injury. The lobster also molts a part of the lining of the digestive tract as far as the posterior portion of the stomach. Immediately after molting the lobster is in a helpless condition, and is more or less at the mercy of its enemies until the new shell, which is secreted by the skin, has grown. This process takes several weeks to complete.

Economic Importance. The lobster is highly esteemed as food, and is rapidly disappearing from our coasts as the result of overfishing. Between twenty million and thirty million are yearly taken on the North Atlantic coast. This means a value at present prices of about $15,000,000. Laws are now enacted in New York

and other states against overfishing. Egg-carrying lobsters must be returned to the water, all smaller than six to nine inches in length (the law varies in different states) must be put back; other restrictions are placed upon the taking of the animals, in hope of saving the race from extinction. Some states now hatch and care for the young for a period of time; the United States Fish Commission is also doing much good work, in hope of restocking to some extent the now almost depleted waters.

In

Shrimps. Several other common crustaceans are near relatives of the crayfish. Among them are the shrimps and prawns, thinshelled, active crustaceans common along our eastern coast. spite of the fact that they form a large part of the food supply of many marine animals, especially fishes, they do not appear to be

toms; in such localities they are caught in great numbers in nets or traps baited with decaying meat. They are, indeed, among our most valuable sea scavengers, although they are carnivorous hunters as well. The body of the crab is short and broad, being flattened dorso-ventrally. The abdomen is much reduced in size. Usually it is carried close to the under surface of the cephalothorax; in the female the eggs are carried under its ventral surface, fastened to the rudimentary swimmerets. The young crabs differ considerably in form from the adult. They undergo a complete metamorphosis

(change of form), and their method of life differs from the adult. Immediately after molting, crabs are greatly desired by man as an article of food. They are then known as "shedders," or soft-shelled crabs.

OTHER CRABS. Other crabs seen along the New York coast are the prettily colored lady crabs, often seen running along our sandy beaches at low tide; the fiddler crabs, interesting because of their burrows and gregarious habits; and perhaps most interesting of all, the hermit crabs. The hermit crabs use the shells of snails as homes. The abdomen is soft,

and unprotected by a limy exoskeleton, and has adapted itself to its conditions by curling around in the spiral snail shell, so that it has become asymmetrical. These tiny crabs are great fighters and wage frequent duels with each other for possession of the more desirable shells. They exchange their borrowed shells for larger ones as growth forces them from their first homes.

The habits of these animals, and those of the fiddler crabs, might be studied with profit by some careful boy or girl who spends a summer at the seashore and has the time and inclination to devote to the work. Of especial interest

would be a study of the food and feeding habits of the fiddler crabs.