plasm begin to break up by repeated division (the parts all being held together, however, in the wall of the zygote), and by the end of the twelfth or fourteenth day the zygote's protoplasm may have become divided into ten thousand minute sporozoites. The zygote wall now breaks down, thus releasing the thousands of active little sporozoites into the general body cavity of the mosquito. This cavity is filled with flowing blood plasm-insects do not have a closed but an almost completely open circulatory system-and swimming about in this plasm the sporozoites soon make their way forward and into the salivary glands of the mosquito. Now when the insect pierces a human being to suck blood, it injects a certain amount of salivary fluid into the wound (presumably to keep the blood from clotting at the puncture) and with this fluid go many of the sporozoites. Thus a new infection of malaria is made. The sporozoites may lie in the salivary glands for several weeks, and so for the whole time from twelve to fourteen days after the mosquito has become infected with the malarial parasite by sucking blood from a malarial patient until the sporozoites in the salivary glands finally die, it is a means of the dissemination of the disease. There can be no malaria without mosquitoes to propagate and disseminate it, and yet no mosquitoes can propagate and disseminate malaria without having access to malarial patients. The only mosquito species in this country which has been proved to be a malaria disseminator is Anopheles maculipennis, a spotted-winged form spread over the whole continent.

In the great branch or phylum of flatworms (Platyhelminthes), that group of animals which of all the principal animal groups is widest in its distribution, perhaps a majority of the species are parasites. Instead of being the exception, the parasitic life is the rule among these worms. Of the three classes into which the flatworms are divided, almost all of the members of two of the classes are parasites. The common tapeworm (Tania) (Fig. 212), which lives parasitically in the intestine of man, is a good example of one of these classes. It has the form of a narrow ribbon, which may attain the length of several yards, attached at one end to the wall of the intestine, the remainder hanging freely in the interior. Its body is composed of segments or serially arranged parts, of which there are about eight hundred and fifty altogether. It

has no mouth nor alimentary canal. It feeds simply by absorbing into its body, through the surface, the nutritious, already digested liquid food in the intestine. There are no eyes nor other special sense organs, nor any organs of locomotion. The body is very degenerate. The life history of the tapeworm is interesting, because of the necessity of two hosts for its completion. The eggs of the tapeworm pass from the intestine with the excreta, and must be taken into the body of some other animal in order to de

velop. In the case of one of the several species of tapeworms that infest man, this other host must be the pig. In the alimentary canal of the pig the young tapeworm develops and later bores its way through the walls of the canal and becomes imbedded in the muscles. There it lies, until it finds its way into the alimentary canal of man by his eating the flesh of the pig. In the intestine of man the tapeworm continues to develop until it becomes full grown.

In a lake in Yellowstone Park the suckers are infested by one of the flatworms (Ligula) that attains a size of nearly one fourth the size of the fish in whose intestines it lives. If the tapeworm of man attained such a comparative size, a man of two hundred pounds' weight would be infested by a parasite of fifty pounds' weight.

Another group of animals, many of whose members are parasites, are the roundworms or threadworms (Nemathelminthes). The free-living roundworms are active, wellorganized animals, but the parasitic kinds all show a greater or less degree of degeneration. One of the most terrible parasites of man is a roundworm called Trichina spiralis (Fig. 213). It is a minute worm, from one to three millimeters long, which in its adult condition lives in the intestine of man or of the pig or other mammals. The young are born alive and bore through the walls of the intestine. They migrate to the voluntary muscles of the hosts, especially those of the limbs and back,

and here each worm coils itself up in a muscle fiber and becomes inclosed in a spindle-shaped cyst or cell (Fig. 213). A single muscle may be infested by hundreds of thousands of these minute worms. It has been estimated that fully one hundred million encysted worms may exist in the muscles of a "trichinized" human body. The muscles undergo more or less degeneration, and the death of the host may occur. It is necessary,

for the further development of the worms, that the flesh of the host be eaten by another mammal, as the flesh of the pig by man, or the flesh of man by a pig or rat. The Trichino in the alimentary canal of the new host develop into active adult worms and produce new young.

In the Yellowstone Lake the trout are infested by the larvæ or young of a roundworm (Bothriocephalus cordiceps) which reach a length of twenty inches, and which are often found stitched, as it were, through the viscera and the muscles of the fish. The infested trout become feeble and die, or are eaten by the pelicans which fish in this lake. In the alimentary canal of the pelican the worms become adult, and parts of the worms containing eggs escape from the alimentary canal with the excreta. These portions of worms are eaten by the trout, and the eggs give birth to new worms which develop in the bodies of the fish with disastrous effects. It is estimated that for each pelican in Yellowstone Lake over five million eggs of the parasitic worms are discharged into the lake.

The young of various carnivorous animals are often infested by one of the species of roundworms called "pup worms" (Uncinaria). Recent investigations show that thousands of the young or pup fur seals are destroyed each year by these parasites. The eggs of the worm lie through the winter in the sands of the breeding grounds of the fur seal. The young receive them from the fur of the mother and the worm de

velops in the upper intestine. It feeds on the blood of the young seal, which finally dies from anæmia. On the sand beaches of the seal islands in Bering Sea there are every year

FIG. 214.-Fur-seal pups killed by the parasitic worm, Uncinaria, on the sands of Tolstoi rookery, St. Paul's Island, Pribilof Group. (Photograph by C. H. Townsend.)

thousands of dead seal pups which have been killed by this

parasite (Fig. 214). On the rocky rookeries, the young seals are not affected by this parasite.

Among the more highly organized animals the results of a parasitic life, in degree of structural degeneration, can be more readily seen. A well-known parasite, belonging to the Crustacea-the class of shrimps, crabs, lobsters, and crayfishes-is Sacculina. The young Sacculina (Fig. 215, A) is an active, freeswimming larva much like a young prawn or young crab. But the adult bears absolutely no resemblance to such a typical crustacean as a crayfish or crab. The Sacculina after a short

FIG. 215.-Development of the parasitic crustacean, Sacculina carcinus: A. Naplius stage; B, cypris stage; C, adult attached to its host, the crab, Carcinus manas. (After Hertwig.)

period of independent existence attaches itself to the abdomen. of a crab, and there completes its development while living as a parasite. In its adult condition (Fig. 215, C) it is simply a great tumorlike sac, bearing many delicate rootlike suckers which penetrate the body of the crab host and absorb nutriment. The Sacculina has no eyes, no mouth parts, no legs, or other appendages, and hardly any of the usual organs except reproductive organs. Degeneration here is carried very far.

Other parasitic crustacea, as the numerous kinds of fish lice (Fig. 216) which live attached to the gills or to other parts of fish, and derive all their nutriment from the body of the fish, show various degrees of degeneration. With some of these fish lice the female, which looks like a puffed-out worm, is attached to the fish or other aquatic animal, while the male, which is perhaps only a tenth of the size of the female, is permanently attached to the female, living parasitically on her.