Trichinella spiralis is a small worm parasitic in man, pigs, rats, and other mammals. In the adult stage the worm is an intestinal parasite of the host. The female with ripe eggs does not remain in the lumen, but penetrates into the walls of the intestine, and there lays a great number of embryos. The latter become dis seminated throughout the body, penetrate into muscle fibres and form cysts or capsules. Man becomes infected by eating raw in fected pork; and pigs or rats become infected by eating the remains of other infected pigs or rats. When taken into the intes tine of the new host, the capsules are digested, the small worms are liberated, and grow into male and female Trichinellas.
Filaria bancrofti is a round worm, the adult stages of which live in the lymphatic glands and ducts of man. The female is viviparous and lays an enormous number of embryos, which pass into the circulatory blood system. At regular intervals during the night, these embryos appear in the peripheral blood vessels. They become very numerous from about midnight to i A.M. and then gradually disappear, to reappear the following night. The parasite is transmitted from man to man by a night biting mosquito. The latter while sucking blood of an infected man, takes up the em bryos, which perforate the mosquito's gut, migrate into its wing muscles, grow, leave the muscles and migrate into various parts of its body. When man is bitten by the infected mosquito, the parasites rupture its proboscis, penetrate through the skin of man to the lymphatics where they grow into male and female Filarias.
without producing serious damage. A black stork, for instance, harbouring several hundreds of worms of six different genera invading the lungs, trachea, oesophagus, stomach and intestine did not show any signs of distress. In an apparently healthy two-year old horse were found several thousand helmiriths belonging to seven different genera. A rat heavily infected with Trypanosoma lewisi may show no symptoms of infection. On the other hand, a man dying from African sleeping sickness produced by Try panosoma gambiense shows very few parasites in his blood, and the same applies to animals dying from the "nagana" disease caused by Trypanosoma brucei. In man, bacterial infections are usually responsible for acute disease, which, when overcome, ends in the complete recovery of the host and the disappearance of the parasites. The host, moreover, often acquires properties pro tecting him from further attack by the same kind of parasite. Ani mal parasites, on the contrary, cause a chronic disease which grad ually undermines the health and considerably lowers the vitality of the host. The mechanism of the action of parasites varies with the species of parasite and with its localization in the host. The action may be purely mechanical, consisting in the blocking of some channels, ducts or vessels. This is the case in malignant malaria, where the parasites have a tendency to clog together and obliterate the blood capillaries of the brain and of other organs. Other parasites destroy the cells of their host, as is the case with the malarial parasites which live upon the red blood corpuscles. The same applies to the intercellular stages of Coccidia, Leish mania and other Protozoa, as well as to the spined eggs of the Bilharzial parasites migrating through the tissues of the host. Animal parasites often liberate substances which are distinctly toxic to the host. The severe anaemia of man infected with hook worm and other intestinal worms is due to the absorption of toxic substances produced by these worms. In some cases parasites are known to produce more or less pronounced castration of their host. The sexual glands of the latter become atrophied and the host often acquires the secondary sexual characters of the oppo site sex. Striking examples parasitic castration are shown by crabs harbouring Sacculina and by wasps attacked by Stylops.