and often is reduced to a mere bag through which the fluid food prepared by its host is absorbed. Such animals as have lost power to move about freely, or are otherwise changed by their surroundings, are said to have degenerated.

Sometimes a complicated life history has arisen from their parasitic habits. Such is seen in the life history of the liver fluke, a flatworm which kills sheep, and in the tapeworm.

Cestodes or Tapeworms. These parasites infest man and many other vertebrate animals. The tapeworm (Tania solium) passes through two stages in its life history, the first within a pig, the second within the intestine of man. The eggs of the worm are taken in with the pig's food. The worm develops within the intestine of the pig, but soon makes its way into the muscles. If man eats pork containing these worms, he may become a host for the tapeworm. Another common tapeworm parasitic on man lives part of its life as an embryo within the muscles of cattle. The adult worm consists of a round headlike part provided with hooks, by means of which it fastens itself to the wall of the intestine. This head now buds off a series of segmentlike structures, which are practically bags full of eggs. These structures, called proglottids, break off from time to time, thus allowing the eggs to escape. The proglottids have no separate digestive systems, but the whole body surface, bathed in digested food, absorbs it and is thus enabled to grow rapidly.

Roundworms. Still other wormlike creatures called roundworms are of importance to man. Some, as the vinegar eel found in vinegar, or the pinworms parasitic in the lower intestine, particularly of children, do little or no harm. The pork worm or trichina, however, is a parasite which may cause serious injury. It passes through the first part of its existence as a parasite in a pig or other vertebrate (dog, cat, ox, or horse), where it encysts itself in the muscles of its hosts. 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 and often fatal disease known as trichinosis.

The Hookworm. The discovery by Dr. C. W. Stiles of the

Bureau of Animal Industry, that the laziness and shiftlessness of the whites" of the South is partly due to a parasite called the hookworm, reads like a fairy tale.

The people, largely farmers, become infected with a larval stage of the hookworm, which develops in moist earth. It enters the body usually through the skin of the feet, for children and adults alike, in certain localities where the disease is common, go barefoot to a considerable extent.

A complicated journey from the skin to the intestine now fol

disease.

lows, the larvæ passing through the veins to the heart, from there to the lungs; here they bore into the air passages and eventually reach the intestine by way of the windpipe. One result of the injury of the lungs is that many thus infected are subject to tuberculosis. The adult worms, once in the food tube, fasten themselves and feed upon the blood of their host by puncturing the intestine wall. The loss of blood from this cause is not sufficient to account for the bloodlessness of the person infected, but it has been discovered that the hookworm pours out a poison into the wound which pre

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.

“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.
Sedgwick and Wilson, General Biology. Henry Holt 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.