of its host. In the case of pork, if the meat is eaten in an uncooked condition, the cyst is dissolved off by the action of the digestive fluids, and the living trichina becomes free in the intestine of man. Here it bores its way through the intestine walls and enters the muscles, causing inflammation there. This causes a painful disease known as trichinosis.

Some roundworm parasites live in the skin, and others live in the intestines of the horse. Still others are parasitic in fish and insects, one of the commonest being the hair snake, often seen in country brooks.

CLASSIFICATION OF WORMS (ANNULATA)

CLASS I. Chatopoda (bristle-footed). Segmented worms having setæ.

SUBCLASS I. Polychata (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. Hematoda.

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 and
Trichina.

REFERENCE BOOKS

FOR THE PUPIL

Davison, Practical Zoology, pages 150-161. American Book Company.
Herrick, Text-book in General Zoology, Chap. IX. American Book Company.
Jordan, Kellogg, and Heath, Animal Studies, VI. D. Appleton and Company.

FOR THE TEACHER

Darwin, Earthworms and Vegetable Mould. D. Appleton and Company.
Sedgwick and Wilson, General Biology. Henry Holt and Company.

XVIII. CRUSTACEANS

The Crayfish (Cambarus affinis). - Crayfish live in fresh-water lakes and streams. There they may be caught under projecting stones in clear streams by hand. From muddy streams they may be taken by means of a weighted net, which is pulled along the bottom. Although they prefer the water, they are sometimes found at some distance from any large body of water; such animals are supposed to be migrating.

Crayfish; A., antennæ; E., stalked eye; C.P., cephalothorax; Ab., abdomen; C.F., caudal fin; M., mouth; Ch., chelipeds. From photograph loaned by the American Museum of Natural History.

The Structure and Activities of the Crayfish. Living crayfish in dishes of water should be provided for this exercise. Notice the color of the living crayfish. In the natural habitat the colors blend with its surroundings, so that it is difficult to distinguish a crayfish from the bottom on which it rests. The animal is thus said to be protectively colored. The body is composed of a series of rings or segments. apparent at he anterior end of the body where the head and middle region (thoracic region) are covered by one piece of the skeleton. This is called the cephalothorax. Count the number of segments in the abdomen (the posterior region). This number is constant for every crayfish.

This fact is not

The shell is a true exoskeleton, that is, it is formed by the skin. As in the exoskeleton of the insects, an animal material called chitin forms the basis, but in this case the skeleton is strengthened by the addition of lime. Test a piece of the shell with acid. What results? Is anything left 1 For full laboratory directions see Hunter and Valentine, Manual, page 121.

behind? What is this, animal or mineral matter? Examine the shell of a dead crayfish and try to find out how the segments of the abdomen are joined together. Notice the different positions taken by the abdomen in the living animal. How might this be accounted for mechanically?

The small appendages attached to the under surface of the segments are called the swimmerets or pleopods. How many are attached to each segment? Observe the movement when in the water. Why are they called swimmerets? Notice that each pleopod is made up of three pieces, a base and two branches. Now look at the broad appendage that, together with the last segment of the abdomen, forms a broad finlike apparatus, the caudal fin. You will find this appendage likewise composed of three pieces; it is homologous to the pleopods. This appendage is known as the uropod. Crayfish normally swim very rapidly by means of a sudden jerking in a backward direction of the caudal fin. Other methods of locomotion may be observed by allowing the animal to walk in a shallow pan with a little water in it. Note the position and number of jointed legs that are used in locomotion. Is the large pair of pincher legs used for this purpose? If so, to what extent? Crayfish have a definite method in the movement of the walking legs, the first and third moving in alternation with the second and

Female lobster, showing eggs attached to the swimmerets. From photograph loaned by the American Museum of Natural History.

fourth legs on the same side of the animal. Test a crayfish to see if this rule holds true. Do you find any other methods of locomotion than the ones mentioned?

Watch the animal in its movements to see if, in avoiding objects, it first allows certain parts of the body to touch the object. The longer of the two pairs of feelers (the antenna) function as organs of touch. Hairs which are sensitive to touch are also found in various other parts of the body. The bases of the antennæ are broad, and a small flat piece projects outward from this basal portion. The antennæ are believed to have also the function of smell. Crayfish are thus able to learn of the presence of food at a considerable distance away. The short appendages immediately in front of the antennæ are called the antennula (little antennæ). Notice that the four stalks are in reality two branches from one base on each side. Part of this appendage is believed to contain the organ of hearing. Test in any manner that you can the sight of the crayfish. Test with

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. Touch the eye with a pencil; is it freely movable? In what direction? Notice 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, swimmeret; 6 second maxilliped, showing baler; 7, uropod; 8, first maxilliped, showing gill attached.

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 maxilla 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; l, liver; 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.