These grow in the body without forming attachments to its structures, have an independent life of their own, and possess the power of reproduction and generation. Several species infest the human body, some appearing always in the same organ and some in a particular tissue, and appearing oftenest where that tissue is plentiful. Scarcely any portion of the body is exempt from such growths. Their origin is a subject for two suppositions -- that of generative reproduction, and of accidental or spontaneous development of germs that take on modes of life and development characterizing them afterwards. The first supposition is more philosophical, comports more with analogy, and is unquestionably the true theory. The interest attached to these growths, however, is their effect upon the system and cause of disease. Their presence in the system causes morbid phenomena, disordered functional action, and loss of health. The mischief they do in the system depends upon their number, size, rapidity of growth, and species. When numerous or large they imbibe so much nutriment as to rob the system of its necessary sustenance. Their habitation is generally a seat of irritation or inflammation, and more particularly when their location is in a cavity, and when they possess power of motion.

Psychodiara. -- Hydatids. -- These are organized beings, consisting of a globe-like bag of albuminous matter; the texture divided in layers, and containing a limpid, colorless fluid richer in gelatin than albumen. They live by inbibition, have no sensibility or power of motion, and appear more like a vegetable than an animal in their modes of life and reproduction. There are two kinds of hydatids, the acephalocyst, or cyst without a head, and the echinococcus, which is not different from the other in form but in containing minute animals (vermiculi echinococci) within it. The former is common to the human body, and generated between layers of membrane. The usual abode of hydatids is in the lungs, liver, ovaries, spleen, kidneys, etc. The hydatids occurring in the womb are often mistaken upon expulsion for products of conception, and their presence in that organ often produces similar signs as in pregnancy.

Sterelmintha. -- These consist of solid porous texture, perforated by canals or cavities, which serve the purposes of digestion. These animals are hermaphrodite, i.e., having both sexual organs on one individual. The varieties of tapeworm belong to this class. So also the cysticercus, which occurs in the muscular structure and in the watery portion of the eye. The liver-fluke -- distoma hepaticum -- also belongs to this class, but which rarely occurs in man, but is supposed to cause the well-known "rot" in sheep.

Coelelmintha. -- This class has a higher organic development than the preceding. It embraces several species of worms having hollow cylindrical bodies, distinct alimentary canals, with a mouth at one extremity and an anus at the other, a nervous system, and the sexual organs on different animals. The common intestinal worms belong to this class; so also the trichina spiralis -- causing the disease described below which is an animal which exists within the minute, white, ovate cysts imbedded in the muscles. The guinea-worm (filaria raedinensis), so common to Africa and Asia, but unknown in this country, is a hair-like worm developed beneath the skin, especially in the scrotum and lower extremities. It can be withdrawn when a pustule ensues, by care and patience, wrapping it around a stick until the end appears.

The strongulus gigas is an animal that locates itself exclusively in the kidney, and sometimes attains an enormous size. Its body is round, but tapers toward both ends. It sometimes attains a length of three feet, and a thickness of half an inch. It causes impairment of functions, waste of the renal structure, and sometimes inflammation, with pain and bloody urine. It is sometimes expelled through the water passages.

TREATMENT. -- The treatment of parasites is indicated by their character or place of abode. If they exist in the alimentary canal, such remedies as are known to expel them should be employed. Anything is a good remedy that is harmless to the system but destructive of life to them. Various agents are poisonous, such as cherry-laurel water, camphor, oil of cubebs, oil of turpentine, copaiba, etc., but these must be cmployed at proper seasons and in such quantities that they will not harm the general system.