vents the blood from coagulating (see page 367) rapidly; hence a considerable loss of blood occurs from the wound after the worm has finished its meal and gone to another part of the intestine. The cure of the disease is very easy; thymol, which weakens the hold of the worm, being followed by Epsom salts. For years the entire South undoubtedly has been retarded in its development by this parasite, and hundreds of millions of dollars and, what is more vital, thousands of lives, have been needlessly sacrificed. 66 The hookworm is not a bit spectacular: it doesn't get itself discussed in legislative halls or furiously debated in political campaigns. Modest and unassuming, it does not aspire to such dignity. It is satisfied simply with (1) lowering the working efficiency and the pleasure of living in something like two hundred thousand persons in Georgia and all other Southern states in proportion; with (2) amassing a death rate higher than tuberculosis, pneumonia, or typhoid fever; with (3) stubbornly and quite effectually retarding the agricultural and industrial development of the section; with (4) nullifying the benefit of thousands of dollars spent upon education; with (5) costing the South, in the course of a few decades, several hundred millions of dollars. More serious and closer at hand than the tariff; more costly, threatening, and tangible than the Negro problem; making the menace of the boll weevil laughable in comparison - it is preeminently the problem of the South." Atlanta Constitution. Parasitic worms are of vital importance to mankind. Not only do they levy a tax of death and illness on man himself, but they destroy as well unestimated millions of dollars' worth of animals. Of the 2,000,000 persons infected with hookworm, 500,000 are wage earners (and this is a small estimate); their earnings at $1.50 a day would amount to about $225,000,000 a year. If their wage-earning capacity were decreased only 10 per cent, it is seen that a loss of over $20,000,000 a year could be directly attributed to this pest. Other Parasitic Worms. Some roundworm parasites live in the skin, and others live in the intestines of the horse. Still others are parasitic in fish and in insects, one of the commonest being the hair snake, often seen in country brooks. CLASSIFICATION OF SEGMENTED WORMS (ANNULATA) CLASS I. Chatopoda (bristle-footed). Segmented worms having setæ. SUBCLASS I. Polychaeta (many bristles). Having parapodia and usually head and gills. Example: sandworm. SUBCLASS II. Oligochata (few bristles). No parapodia, head, or gills. Example: earthworm. CLASS II. Discophora (bearing suckers). No bristles, two sucking disks present. Example: leech. PLATYHELMINTHES (FLATWORMS) Body flattened in dorso-ventral direction. CLASS I. Turbellaria. Small, aquatic, mostly not parasitic. Example: planarian worm. CLASS II. Trematoda. Usually parasitic worms which have complicated life history. Example: liver fluke of sheep. CLASS III. Cestoda. Internal parasites having two hosts. Example: tape worm. NEMATHELMINTHES (ROUNDWORMS) Threadlike worms, mostly parasitic. Examples: vinegar eel, Trichina, and hook worm. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Company. Davison, Practical Zoology, pages 150-161. American Book Company. Herrick, Textbook in General Zoology, Chap. IX. American Book Company. Jordan, Kellogg, and Heath, Animal Studies, VI. D. Appleton and Company. Ritchie, Primer of Sanitation. World Book Company. ADVANCED Darwin, Earthworms and Vegetable Mold. D. Appleton and Company. XVIII. THE CRAYFISH. A STUDY OF ADAPTATIONS Problem XXIX. A study of the idea of adaptations as shown in the crayfish (optional). (Laboratory Manual, Prob. XXIX.) (a) Protection. (b) Locomotion. (c) Surroundings. (d) Feeding. (e) Breathing. Adaptations. Plants and animals are in a continual struggle to hold the places they have obtained upon the earth. Continually we see garden plants driven out or killed by the competing weeds, simply because the weeds are better fitted or adapted to live under the conditions which exist in the garden, especially if it is uncultivated. An adaptation in a plant or animal is some structure, habit, or ability which is of advantage to the organism in its battle for life. We have seen many examples of adaptations in plants, adaptations in flowers for securing cross-pollination, in fruits for seed-scattering, in young plants for protection, in roots for watersecuring; the list is endless. In animals, likewise, the successful competitors are the ones with adaptations to fit them for living in the particular environment or surroundings in which nature has put them. Examples are often seen where animals, like sheep or goats, which have a woolly covering, when introduced by man into a warmer country, die because the outer coat is too warm. An adaptation for withstanding cold becomes harmful to the animal under conditions of greater heat. One adaptation which we have already noticed in animals is always protective. This is resemblance of the animal to the surroundings in which it lives. Other adaptations aid the animal in obtaining and digesting food, in protecting itself or its young from attack by enemies, and in many other ways aiding the animal to battle successfully with the dangers around it. The Crayfish. Adaptations for Protection. An animal which well illustrates adaptation for life in the water is the fresh-water crayfish or the salt-water lobster, both members of a large group of animals known as crustaceans. The body of such an animal is seen to be covered more or less completely with a hard covering, which is jointed in the posterior region. This exoskeleton (outside skeleton) is composed largely of lime, as may be proved by testing with acid. The exoskeleton fits over the anterior part of the animal, forming an unjointed carapace, or armor. This armor is clearly protective and is thus an adaptation. If the crayfish is watched in a balanced aquarium, the colors, too, are seen to blend remarkably with the stones and water weeds of the bottom. The animal is protectively colored. The under side of the animal is seen to be less well pro : Crayfish A., antennæ; E., stalked eye; C.P., cephalothorax; Ab., abdomen; C.F., caudal fin; M., mouth; Ch., chelipeds. From photograph. tected than the upper, and the joints of the abdomen, or posterior region, are seen to extend completely around the body. The animal is thus seen to be segmented, the abdomen showing this plainly. The seven segments in the abdomen are constant for every crayfish. Locomotion. Those of us who have caught crayfish in fresh-water streams or lakes know that it takes skill and quickness. They dart backwards through the water with great rapidity, or they may move forward by crawling on the bottom. Examination of a crayfish shows us five pairs of walking legs attached to the under side of the cephalothorax (head + thorax), the anterior part of the body. These legs are jointed, the first three bearing pinchers. The large pincher claw is used partly for food-catching, and for locomotion as well. Try to find out, in a living specimen, exactly what part it plays. Under the abdomen, one to each segment except the last, are found jointed appendages, made up of three parts, a base and two branches. These are called swimmerets, though they are not used for swimming. Now look at the broad pair of appendages that, together with the last segment of the abdomen, form a finlike apparatus, the caudal fin. The caudal fin is composed of two large swimmerets and the last body segment. Crayfish normally swim very rapidly by means of a sudden jerking in a backward direction of the caudal fin. The abdomen is provided with powerful muscles which are attached to the exoskeleton. It is by these muscles that the rapid swimming is accomplished. How the Crayfish gets in Touch with its Surroundings. Several other appendages besides those used for locomotion are found. Two pairs of "feelers," the longer pair called the antenna, the shorter the antennules (little antennæ), protrude from the front of the body. The longer feelers appear to be used as organs of touch. Certain hairlike structures projecting from the antennæ have to do with the sense of smell. The smaller antennules hold at their bases little sacs called ears." These " ears" have largely to do with the function of balancing rather than hearing. 66 Female lobster, showing eggs attached to the swimmerets. From photograph loaned by the American Museum of Natural History. Just above the antennules, projecting on stalks, are the eyes. These eyes are made up of many small structures called ommatidia, each of which is a very simple eye. A collection of ommatidia is known as a compound eye. A little bit of the outer covering of the eye, mounted under a compound microscope, shows these eye units to be shaped like tiny rectangles in cross section. Such an eye probably does not have very distinct vision at a distance. A crayfish, however, easily distinguishes moving objects and prefers darkness to light, as may be proved by experiment. Feeding. If it is possible to have the aquarium holding the crayfish in the schoolroom, the method of feeding may be watched. 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 three pairs of small appendages called foot jaws (maxillipeds), and to a |