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sides this value as a food, they are also used by man, the shrimp fisheries in this country aggregating over $1,000,000 yearly.

The Blue Crab. Another edible crustacean of considerable economic importance is the blue crab. Crabs are found inhabiting muddy bottoms; 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 in the position which is usual for other crustaceans. 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.

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Other Crabs.

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The edible blue crab. From photograph loaned by the American Museum of Natural History.

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.

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Hermit crab, about twice natural size. From photograph loaned by the American Museum of Natural History.

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 con

ditions 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.

A deep-water crab often seen along Long Island Sound is the spider crab, or "sea spider," as it is incorrectly called by fishermen. This animal, with its long spiderlike legs, is neither an active runner nor swimmer; it is, however, colored like the dark mud and stones over which it crawls; thus it is enabled to approach its prey without being noticed. The resemblance to the bottom is further heightened by the rough body covering, which gives a hold for seaweeds and sometimes sessile animals, as barnacles, hydroids, or sea anemones, to fasten themselves.

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Giant spider crab from Japan. From photograph loaned by the American Museum of Natural History.

Symbiosis.

A spider crab from the Sea of Japan is said to be the largest crustacean in the world, specimens measuring eighteen feet from tip to tip of the first pair of legs having been found.

Certain of the spider crabs, as well as some of the larger deep-water hermit crabs, have come to live in a relation of mutual helpfulness with hydroids, sponges, and sea anemones. These animals attach themselves to the shell of the crab and are carried around by it, thus receiving a constant change of position and a supply of food. What they do for the crab in return is not so evident, although one large Chinese hermit regularly plants a sea anemone on its big claw; when forced to retreat into its shell, the entrance is thus effectually blocked by the anemone. The

living of animals in a mutually helpful relation has been referred to as symbiosis. Of this we have already had some examples in plants as well as among animals. (See page 187.)

Habitat. - Most crustaceans are adapted to live in the water; a few forms, however, are found living on land. Such are the wood lice, the pill bugs, which have the habit of rolling up into a ball to escape attack of enemies, the beach fleas, and others. The coconut crab of the tropics climbs trees in search of food, returning to the water at intervals to moisten the gills.

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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, jointed appendages, usually a pair to each segment of the body (except the last), stalked compound eyes, and the fact that 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 varies, 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.

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class of crustaceans, in which the number of appendages varies, is the group Entomostraca. They are mostly small animals, some species existing in countless numbers. One of the largest Entomostracans inhabiting fresh water is the fairy shrimp (branchippus) found appearing in early spring in freshwater ponds, a little translucent swimming animal from one half to three fourths of an inch in length. Another fresh-water form often seen in aquaria is the water flea (daphnia). From the economic standpoint, probably the most important crustaceans that we shall study are the copepods. These tiny

Cyclops (note the single eye-spot). This is a very common copepod and is magnified about forty times. e, egg masses.

animals are barely visible to the naked eye. They are found in almost every part of the world, from the arctic seas to those of the tropics, and in fresh as well as salt water. They are so numerous that the sea in places is colored by their bodies. So prolific are they that it is estimated that one copepod may produce in a single year four billion five hundred million offspring. These animals form a large part of the food supply of many of our most important food fishes as well as the food of many other aquatic animals. The whale, for example, subsists largely on this kind of food. They are, then, in an indirect way, of immense economic value.

Degenerate Crustaceans. One of the most interesting forms to a zoologist is the goose barnacle. This crustacean, like all others of the group, is free-swimming during its early life. Later, however, after passing through several changes in form during its development, the barnacle settles down on a rock or some floating object, fastens itself along the dorsal surface, and remains fastened during the rest of its life. Food comes to it in a current of water, which is set in motion by the rhythmical beating of the appendages. Thus food particles are carried along the ventral side of the body to the mouth. Such animals, having lost the power of locomotion, are said to be degenerate.

Parasitic Crustaceans. Other crustaceans have become even more helpless and have come to take their living from other animals. In some cases they become simply a bag for absorbing nourishment from the host on which they are fastened. Such is the sacculina, a degenerate crustacean that lives attached to the body of the crab. Others attach themselves to fishes and are known to fishermen as fish lice.

REFERENCE BOOKS

ELEMENTARY

Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company.

Burnet, School Zoology, pages 67-73. American Book Company.

Davison, Practical Zoology, pages 133–141. American Book Company.

Herrick, Textbook in General Zoology, Chap. XIII. American Book Company. Jordan and Kellogg, Animal Life, Chap. VIII. D. Appleton and Company. Jordan, Kellogg, and Heath, Animal Studies, Chap. IX. D. Appleton and Com

pany.

ADVANCED

Herrick, The American Lobster, Report of U.S. Fish Commission, 1895.

Huxley, The Crayfish. D. Appleton and Company.

Mead, Reports of the R.I. Inland Fisheries Commission.

Parker, Elementary Biology. The Macmillan Company.

Parker and Haswell, Textbook of Zoology. The Macmillan Company.

XIX. THE INSECTS

Problem XXX. A study of some animal likenesses and differences, and the classification of insects (optional). (Laboratory Manual, Prob. XXX)

(a) Grasshopper-a straight-winged insect.

(b) Butterfly or moth― a scale-winged insect.
(c) The typhoid fly- a two-winged insect.
(d) A beetle-a sheath-winged insect.
(e) A bug-a half-winged insect.

(f) The dragon fly-a nerve-winged insect.
(g) The bee-a membrane-winged insect.
(h) Summary of differences between orders.
(i) Making a logical definition.

Insects the Winners in Life's Race. We are all familiar with common examples of insect life. Bees and butterflies we have already studied in connection with their work in the cross-pollination of flowers. Mosquitoes and flies all too often come to our notice as pests; the common household insects sometimes annoy us, while we often hear and see in a small way the harm done by insects in the field and garden. Insects are a successful group. They outnumber all the other species of animals on the face of the earth. They hold their own in the air, in the water, and on land. Fitted in many ways to lead the successful life, they have become winners in life's race.

We have already, from our study of a bee, formed some idea of what an insect is. But it would be unfair to expect to know all insects from our slight knowledge of one form. Our object in the study of this chapter will be to get some first-hand knowledge of some common insects so that we may classify them and distinguish one from another. This great group, containing more than half of the known representatives of animal life on the earth, is made up of a number of groups called orders. The insects contained in these orders have certain characters of structure and

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