ENTOZO'A. This term is applied to all the animal forms which live either in the natural cavities (as, for example, the intestinal canal), or in the solid tissues (as, for example, the liver) of other animals. The number of these parasites is so great (there being at least 20 distinct species of worms found in man, 14 in the dog, 15 in the horse, 11 in the common etc.), and their occurrence so frequent, especially in some of the lower animals, that we must regard their presence, at all events in many species, rather as the normal condition, than as a morbid state due to accidental causes.
It is worthy of notice, that many of the animals included amongst- the E. only enjoy a parasitic existence during a part of their total life, which often, as in the well-known case of perfect insects, presents very varied and distinct phases. Thus, for example, the larvm of the gadfly ((estrus equi) undergo their entire development in the stomach of the horse, attaching themselves by minute hooks to the gastric mucous membrane; they then detach themselves, pass along the intestines, and in due time are discharged, and undergo their further changes externally; and many similar instances might be quoted. For this reason, and additionally because parasites are now known to belong to various classes of animals, we no longer attempt, like Linnaeus and Cuvier, to form a special group of E.; and a reference to the vennes intestines in the systema natures, or to the entozoaires in the regale animal, at once shows that these illustrious naturalists grouped together animals with few or no true natural affinities.
Although most E. belong to the class of vermes, or worms, this, as has been already observed, is by no means exclusively the case. Thus, even fishes may lead a parasitic existence; a fish of the genus fierasfer being frequently found in the respiratory cavity of the holothuria tubulosa, or and small fishes having been frequently observed in the cavity of the asteria discoides. Amongst the crustaceans, instances of parasitism are by no means rare; different species of lernea being abundant in the brauchial (or gill) cavity, and on the surface of numerous fishes, while the lin guatulce infest mammals, reptiles, and fishes, being found in the olfactory sinuses, the larynx, the lungs, the peritoneal cavity, etc. The instances in which mollusks are found to live parasitically are few; certain gasteropods, however, inhabit the bodies of echino derms, holothurias, and comatulas; and amongst the lamellibranchiates, species of modiolaria and mytilus live in the bodies of ascidians. There are several cases of polyps which have been observed to adopt a parasitic existence; and finally, various protozoa are not unfrequently met with in the animal fluids; for example, certain spe cies of vibrio, cercomonas, and paramecium, have been found in the intestinal evacua tions in cholera and diarrhea; monads have been found in the urine in cholera, and cer tain infusoria and rhizopoda in the blood of the dog, the frog, and many other animals.
See ILENIATOZOA.
The more common kinds of E. appear to have attracted the notice of the earliest physicians and naturalists whose opinions or works have reached us. Hippocrates speaks of several worms, especially the ttenim and ascarides, infesting the human intes tinal canal; and Pythagoras learned in India that the bark of the pomegranate acted almost as a specific in cases of tape-worm. Aristotle noticed both the tape-worm of the dog and of man, and the cysticercus cellulosic (see CESTOID Wonms) of the pig; but utterly unconscious that the cysticercus, under favorable conditions, became developed into a tape-worm (see TAPE-won*, referred the origin of all intestinal worms to spontaneous generation—a doctrine that seems to have been generally adopted till the 17th c., when Redi published (in 1684) a work on helminthology, in which he distinctly showed that the generation of various E. followed the same laws as in higher animals, and that in many instances there were distinct males and females. The great recent discovery, that the vesicular or bladder-like parasites, such as the differ ent species of cysticercus and ccenurus, are cestoid worms in an early stage of develop ment, is alluded to in CESTOID Wonms, and will be more fully noticed in the article TAPE-WORM.
Another point of general interest in connection with E. is the part of the body in which they are found. While most live in the intestinal canal and other open cavities (as the larynx, bronchial tubes, etc.), others are found in the closed cavities and in the parenchymatous tissue of the liver and other solid organs. Thus (confining our remarks to the E. occurring in man), anehylostoma duodenale, strongylus duodenalis,. two species of ascaris, oxyuris vermicularis, trichocephalus dispar, distoma heterophyes, at least four species of &silk, and bothriocephalus latus, have been found in different parts of the intestinal canal; while strongylus gigas inhabits the kidney, another species of strongylus the lungs, a species of spiroptera the bladder, two species of fi/aria and monostoma lentil the eye, trichina spiralis the voluntary muscles, two species of echinocoecus and eystleer MS cellulosic, various parenchymatous tissues, two species of distoma the gall-bladder, another species the portal vein, and the filecria meclinensis, or guinea worm, the sub cutaneous tissue.
Davaine, who may be regarded as one of the highest living authorities on this sub ject, gives the following synopsis of the E. occurring in man and the domestic animals (see his Trait des Entozoaires, Paris, 1860).