kidneys. As a rule, those which inhabit a temporary host install themselves in a closed organ; in the muscles, the heart, or the lobes of the brain; those, on the contrary, which have arrived at their destination, and which, unlike the preceding, have a family, occupy the stomach, with its dependencies the digestive passages, the lungs, the nasal fossæ, the kidneys, in a word, all the organs which are in direct communication with the exterior, in order to leave a place of issue for their progeny.
A single animal may carry, not only a great number of individuals of the same species, but many different species of parasites, and this, too, without any apparent impairment of health. Indeed, in some countries their presence is considered indispensable to the highest health, the Abyssinians, for example, deeming themselves below par unless they nourish one or many tapeworms. Nathusius speaks of a black stork which lodged 24 Filariœ in its lungs, 16 Syngami tracheales in its tracheal artery, more than 100 Spiropterœ within the membranes of the stomach, several hundred of the Holostomum excavatum in the smaller intestines, 100 of the Distoma ferox in the large intestines, 22 of the Distoma hians in the œsophagus, and a Distoma echinatum in the small intestine. In spite of this affluence of lodgers, the bird did not appear to be the least inconvenienced. Krause, of Belgrade, mentions a colt, two years old, which contained more than 500 Ascarides, 190 Oxyures, 214 Strongyli armati, several million Strongyli tetracanthi, 69 Tœnia, 287 Filariœ, and 6 Cysticerci. Well supplied as these animals appear to have been, when we consider the number of eggs a single worm may produce, the wonder is that parasites are not more numerous than they are: 60,000,000 eggs have been counted in a single nematode, and in a single tapeworm more than 1,000,000,000 eggs have been found!
While nearly all animals, including parasites themselves, are made to contribute to the support of others, those to which man gives food and lodging are of greatest interest, and he is by no means scantily provided with this class of dependents. Four different cestodes, or tapeworms, live in his intestines; three or four Distoma lodge in his liver, intestines, or blood; nine or ten hematodes, or round worms, inhabit his digestive passages or flesh; and cysticerci, echinococci, and hydatids, are also among his guests. He provides a living for three or four kinds of lice, for a bug, for a flea, and two ascarides, without mentioning certain inferior organisms which lurk in the tartar of the teeth, or in the secretions of the raucous membrane of the mouth. Some of these are confined to him exclusively, others may also find a home on the lower mammalia; some make his body their home while passing through a single stage of development, beginning or finishing the process, as the case may be, in the body of another animal; and others, again, are but day-boarders, taking their meals at his expense, and finding lodgings elsewhere.
