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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.

history. Example,

Usually parasitic worms which have complicated life 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.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

Crayfish; A., antennæ; E., stalked eye; C.P., cephalothorax; Ab., abdomen; C.F., caudal fin; M., mouth; Ch., chelipeds. From photograph.

The Structure and Activities of the Crayfish. Living crayfish in dishes of water should be provided for this exercise.1 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. This fact is not apparent at the 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.

As in

The shell is a true exoskeleton, that is, it is formed by the skin. 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.

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.
Jordan, Kellogg, and Heath, Animal Studies, VI.

American Book Company.
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.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

Crayfish; A., antennæ; E., stalked eye; C.P., cephalothorax; Ab., abdomen; C.F., caudal fin; M., mouth; Ch., chelipeds. From photograph.

The Structure and Activities of the Crayfish.-Living crayfish in dishes of water should be provided for this exercise.1 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. This fact is not apparent at the 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.

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.

[graphic]

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?

Hairs

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. 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

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