Epizoa (Gr. upon, and an animal), a terra used by Owen to signify only a singular class of humbly organized articulate animals, which infest the skin, gills, and eyes of marine animals, but now applied to the most important of the external parasites of the animal kingdom. They all belong to the branch articulata, and to the classes Crustacea, arach-nida, and true insecta. Beginning with the first of these, we shall find that, like the entozoa, many of them possess limited powers of locomotion, and consequently must pass the whole term of their existence upon the animals they infest; but that as we ascend in the scale of organization, and come to the arachnida, and especially the insecta, there is no longer this dependence upon a fixed position for sustenance and habitation, and that, more independent of the will of others, they only make use of their hosts for accidental nourishment, or compel them to take charge of their young. We shall consider the most important of them in the order of this classification, referring for their anatomy and general description to the articles respectively devoted to these classes.

I. Crustacea. The parasitic representatives of this class are confined to the poecilopodous entomostraca, and are found only upon marine animals, being in fact the substitutes for insects, which cannot live beneath the water. These are subdivided into the lerneadoe and the siphonostoma, which together formed Owen's class of epizoa. The former of these have for a long time puzzled the naturalist on account of their peculiar appearance. Aristotle and Pliny described them; Linnaeus placed them among the mollusca; Lamarck removed them to the annelides; and Cuvier arranged them among his intestinal worms. Their forms are very various and fantastic, but they are mostly elongated, with tubular necks of a horny consistency, at the end of which is the mouth armed with sharp implements, by which they attach themselves to the eyes, gills, and flesh of fishes, and suck their blood. The females have long plumose appendages attached posteriorly, which are the ovaries. The males are imperfectly known. The young when first hatched are oval, and possess natatory limbs, by aid of which they seek their proper host, and which, when this object is accomplished, are either transformed by metamorphosis into grasping organs, or are lost.

They are often found in great numbers attached to the same fish, and some are even 6 or 8 in. long. They occasionally excite even the largest sword or sun fish to such a state of desperation by the torments they inflict, that it dashes itself upon the beach. They inhabit both fresh and salt water. The siphonostoma are of a higher order. They have an oval, flattened body, which is partially protected by a hard shield or carapace, and are provided with three or four pairs of feet armed with sharp claws, by means of which, and sucking disks, they fix themselves to the skin of fishes, and soft parts of Crustacea and other aquatic animals. Particular species generally infest particular fishes; and scarcely any fish is free from them. They move with considerable rapidity over the body of the fish, and may leave it for another host. The caligi, of which as many as 40 have been removed from a single codfish, are generally found on weak or diseased fishes on the parietes of the mouth and bronchial cavities, but are unable to suck their blood. Fishermen call them fish lice. The cyamus is sometimes found in such numbers upon the whales of the southern ocean as to entirely strip them of their epidermis, and to produce a white color recognized at a considerable distance.

None of the crustaceous parasites are ever found on terrestrial animals. II. Arach-nida. In this class, nearly allied to the insects, we find a body divided into two principal parts, viz., cephalothorax and abdomen, and provided with four pairs of legs. The abdomen may be subdivided into several segments. The only parasites belonging to it are included in the order acarina or mites. These are minute animals, in which the head, thorax, and abdomen are blended in one oval mass. In their immature state they have three pairs of legs; the fourth they acquire later. Before taking up the true mites, we will describe briefly two genera which are found on man, viz.: lingua tula and demodex. The first, sometimes called penta-stomum, has an elongated, cylindrical body, made up of alternate rings and constrictions, and is about half an inch long. Its head is armed with two large hooks resembling the thorn of a rose bush. It is found enclosed in cartilaginous or calcareous cysts on the surface of the liver in negroes. Another species (L. ferox) is now and then met with in post mortem examinations encysted on the surface of the liver of whites, but is oftener found in the frontal sinuses of herbivora and dogs.

The demodex folliculorum bears also the generic names acarus and steazoon, and is the pimple mite or dweller in the follicles of the human nose. As long ago as the middle of the 17th century it was known that an animal inhabited the comedon, a hard inflamed tubercle which appears on the forehead and skin, especially of young men; but not until 1842 was the subject investigated, by Henle and Simon at the same time. The head of this microscopic parasite is separated from its body by a half-moon-shaped constriction, and is furnished with a double-jointed papilla armed with sharp hooks or saws. The four pairs of legs are short, and consist of three joints which move with difficulty, and are tipped according to some authorities with three claws, to others with but . one. Several forms are met with owing to difference of age and sex. First we see one whose lizard-like tail is three times the length of the body; the contents of this extremity are dark and granular, consisting of fat globules. In another form the shape is nearly the same, but the whole animal is smaller, and has but three pairs of legs; this is undoubtedly immature.

Still a third presents itself with a body like that first described, but with a hinder extremity no longer than the body, and of a conical form, displaying transverse chitinous rings. It seems much more plausible to consider this the male, than to suppose that the tails of the former varieties drop off or shorten. No definite internal structure has yet been made out. Wedl and some other observers think they have made out within the body of the female, and in the field, immature forms without extremities; and if this be true, they are viviparous. They are found generally in the hair follicles of the nose of thick and fat-skinned persons, but may yet be met with on the breast or back, or wherever comedones occur, of which, when present in numbers, they may be the cause, although generally they occasion no trouble. They are usually found with their hinder extremity next the surface, and either close to the hair, or in the canals of the fat glands, upon the secretions of which they live. Their occurrence is very general, and to find them, we have only to squeeze the follicles on the sides of the nose between the finger nails, and to add to their contents beneath the microscope a drop of oil, by which the sebaceous matter is rendered clear.

In the dead body they will be found much more deeply seated, as if they had sought warmth by penetrating toward the interior as the surface became cold. The aca-rus or sarcoptes scabiei, or itch insect, will be fully considered in the article Itch. Still other forms of acari or sarcoptes are sometimes met with on man, transferred to him from the beasts on which they live. Their occurrence is purely accidental, and they are never known to reproduce in such situations. The eruption they cause may be of long continuance, but only because fresh infection takes place by continued contact with the animals affected. The sarcoptes of the various domestic quadrupeds produce upon them the disease known as mange, and are specifically different. The mite of the cat and lion, however, resembles and is probably identical with that of man; so that it is a question whether these creatures got their itch first from man or vice versa. The parasite of the horse is large enough to be visible to the naked eye, and its mode of burrowing and of reproduction is nearly the same as that of the sarcoptes hominis.

It produces a dry scaly appearance of the skin, which is sometimes called "scratches." The cheese and dried-fruit mites may likewise live for a short time on the skin, but cause nothing more than a passing irritation. The family of Ixodes, or ticks, is also a great plague to man and beast. They live on moss and dry foliage, on sunny hillsides, and in groves and thickets, and never fail to attack grazing cattle and passers by. They bore into the skin with their sharp proboscis armed with horny barbs, and remain hanging till the body, at first minute and flat, becomes swollen with blood, even to the size of a bean. To tear them away is impossible on account of their recurved barbs, and great caution and patience are necessary; for if violence is used, the head remains behind, and causes inflammation, which may last for months. Generally long and gentle rubbing with some essential oil will make them quit their hold voluntarily. They lay a vast number of eggs, and their multiplication upon oxen and horses is sometimes so great that the animals die of exhaustion. The gama-sida, beetle lice, are other mites parasitic on birds, reptiles, and insects, and both land and water beetles are often found covered with them.

The dermanyssus avium abounds in great quantities in bird cages and hen houses, and lives upon the blood of their inhabitants. Numerous cases are on record of their presence in great numbers on persons who frequent such localities, penetrating and living beneath the epidermis. They produce the disease occasionally met with among the wretched and filthy sick of the poor, called acariasis. Colonies of mice are often infested with similar parasites. Another mite similar to the ixodes is the leptus autumnalis of Europe, which, living in grass or grain or upon fruit bushes, gets upon the reapers and passers by, and causes pustules and sores. It is red, whence the name of the disease, rou-get. A similar parasite is the bete rouge of Martinique, which often renders necessary amputation of the sufferer's limbs. III. Insecta. In this division we shall consider the parasitic insects of animals in order commencing with those of the mammalia. The human body serves as a residence for several of these, the best known and most numerous of which are the pediculidoe, or lice, which belong to the apterous ametabola, or wingless insects without metamorphosis. Four are peculiar to man : Pediculus capitis, P. vestimenti, P. tabescentium, and phthi-rius pubis or inguinalis.

The head louse is grayish white, and it is supposed to adapt itself to the color of the hair of its host. The males are smaller and less numerous than the females. The eggs, which are bean-shaped, cling to the hair as soon as laid, probably by means of some glutinous matter secreted by the female. After remaining as nits for 6 days, the young emerge, and at the end of 18 days more are capable of reproducing. Each female can deposit 50 eggs in all. The presence of lice is easily detected, for we may see them with the naked eye, and their eggs attached to the ends of the hair cannot escape detection. Even when the old are at work beneath the disgusting disease they create, the females creep forth to deposit the nits upon the fine ends of the hair, perhaps because too great heat is prejudicial. A mere itching is the first symptom of lice, which leads in simple cases to scratching and slight excoriations of the scalp. Let heads so infested remain for months uncombed and un-cared for, and such cases will result as are often seen in European hospitals. A specimen is brought in with hair all matted together in flakes, and looking as if sand and molasses had been poured upon it and dried. The stench is loathsome and sickening.

On raising the hair a frightful mass of filth, pus, scabs, and lice is visible. The scalp is found covered with crusts of blood, with open ulcerating sores, and with thick and elevated scabs, from beneath which on pressure pus flows freely. The ears, too, may be converted into a suppurating surface. The P. vestimenti, or body louse, is much larger than the preceding species. The head is longer, and its color dirty white. This animal is seldom if ever found on the body, but inhabits the seams and folds of clothing next the skin, where it deposits its eggs. Its bite causes the same itching as that of the P. capitis, but the results are different. The scratching brings on papules, which become excoriated, and eczema appears. The clothes adhere to the skin, which brings on exudation, and lastly pustules appear. In some cases constant scratching produces such a hyperemia that a deposition of pigment follows sufficient to color the whole skin like that of the negro. The P. tabescentium of writers has longer antennae and a larger and more distinctly separated thorax than the two preceding species, and an indistinctly ringed abdomen. It inhabits the skin itself, living in its fold beneath the epidermis, and produces the disease called phthiria sis.

Leeuwenhoeck cultivated a colony on his own leg for a considerable time, and. estimated that in eight weeks one female might become the grandmother of 5,000. But some of the best authorities deny the existence of any such species. The phthirius pubis is considerably broader, and has a shorter posterior extremity, than its relatives. Its legs are long, and the hindermost two are armed with immense claws. It is very slow in its motion, and has no eyes. This species, as its name implies, is found most frequently on the pubes, but occasionally on the beard, eyebrows, and hair of the breast and axillae, where it bites deeply into the skin, and lives upon the blood of its host. When present in numbers, these parasites cause an intolerable itching, and may be seen sticking firmly to the surface of the body like black specks of coal. Kuchenmeister has found on the heads of an Egyptian mummy and a New Zealand savage nits whose claws differ somewhat in size from those of the ordinary species. Lice are a world-wide pest, and, no nation seems free from them. According to Aristotle, they must have been a great plague among the ancients, and Alcman, Sulla, and Philip II. are reported to have died of them.

But it is probable that some other parasite, as the mites, was confounded with them. - Rising a step higher among the insects, we come to the hemimeta-bola, or those with an incomplete metamorphosis. In the order hemiptera we find the cimex lectularius or acanthia lectu-laria. The bedbug has a small head, from which project two long three-jointed antennae Behind the compound eyes are two small transparent flaps covered with bristles, which are the rudiments of wings. The thorax is broad and short, the abdominal segment very large, broad, and flat. The eggs are long and cylindrical, and are furnished with a stem, by which in the spring the female fixes them upon objects. It is of a reddish brown, and has a very disagreeable odor, which arises from two glands that contain a red and granular matter. This pest inhabits the crevices of beds, walls, and furniture, or wherever it can find a convenient place to conceal itself by day. It will lodge in garments also, but always emerges at night to prey upon the blood of man. Its predatory excursions are not wholly confined to the night, for when present in the clothing it bites as well by day. The skin of some individuals seems quite insensible to their bite, while upon others it causes great local irritation.

The black point seen in the centre of the spot is caused by the coagulation of the blood left in the wound. Sometimes a person is literally almost devoured by these creatures, and the whole body may be covered with the eruption they produce. They are found generally wherever man exists. - The flea (pulex irritans) belongs to the holometabolous apha-niptera, or hopping diptera, which undergo a complete metamorphosis. Its head is short and rounded. The eye is simple. The mouth is provided with two four-jointed palpi, with a long tongue protected above by a short double upper jaw, and a sort of upper double lip or taster, and below by a projecting under jaw. The thorax is provided with two pairs of stigmata, and with three pairs of legs, the first of which are seemingly on the head. The two hindermost are composed of many tarsal joints, which are very long, and furnish the means by which its enormous leaps are taken. They are provided also with long double claws. The posterior segment is covered with ten plates or rings lapping over each other, like shingles on a roof. The color of this parasite is a reddish brown. The male is smaller than the female, and its abdomen is flatter and broader. The eggs are oval, white, and covered with a glutinous matter.

In six days after their deposition, either in dust or beneath the nails, small, worm-like, jointed larvae without feet creep forth from them. In 11 days more they envelope themselves in a thin cocoon, from which at the expiration of 11 days they emerge perfect animals. It is a disputed point whether the males are parasitic; Kuchenmeister argues from the structure of their head that they are not. Little need be said here about the habits of this insect, which in some countries, as Italy, Turkey, and Germany, is such an intolerable nuisance. It bites all the time, day and night, and is never satisfied. Its bite, though productive of more itching, does not cause the great irritation the acanthia produces. Their horny covering or mail protects them from being crushed except by a wonderful degree of pressure, and their alert senses enable them to avoid the hunter's hand, unless it be skilful and experienced. The pulex penetrans, or chigo, jigger, or sand flea, as it is variously called, is smaller than its relative, and has a proboscis longer than its body. (See Chigo.) - The larvae of several forms of insects are occasionally found either in or on some part of man; but as they are an accidental occurrence, they need only be alluded to here.

The larvae of some unknown oestrus are sometimes met with beneath the skin. They form pimples from which flows a moisture, while around them the skin is red and painful. Humboldt in his South American travels met Indians with large parts of their exposed bodies thus affected. In the intestinal canal the larvae of anthomyia scalaris and canicularis are sometimes found. The musca vomitoria, or bluebottle, sometimes deposits its larvae in open cavities of the body, as the ear, eye, or wherever else moisture and heat are found. The common flesh fly, M. carnaria, and the M. domestica, also deposit their eggs at times in hot weather either on open wounds or moist places of the body, and cause the appearance known as "live sores." The larvae are sometimes deposited in a highly developed condition, so that they become maggots even in a few hours. Quadrupeds also are infested by lice, almost without exception each by one peculiar to itself, though sometimes one species is known to live upon several animals of the same genus. They increase with great rapidity upon such beasts as are kept in dirty stables, seldom cleaned, and poorly cared for, and most frequently are seen upon old horses.

They cause irritation, roughness of skin, and loss of hair, in consequence of the disposition of their hosts to bite and rub the affected parts. Fleas abound upon several animals, and are nearly always distinct species. The most frequent and troublesome pests of the herbivora are various oestri (gasterophili) or breeze flies. The oestrus peculiar to the horse, for instance, produces the well known disease called bots. (See Bots.) Another species, OE. ovis, deposits its eggs in the nostrils of sheep, usually about half a dozen in each individual. The larvae are soon hatched, and creep by means of their two anterior hooks upward into the frontal and maxillary sinuses. There they remain until ready to undergo metamorphosis, when they fall out, gain their wings, and repeat the process. The larvae are composed of 12 segments besides the head. Sheep fear these flies greatly, and often huddle together with their heads close to the ground to avoid them. The symptoms of their presence are sneezing and a discharge of glairy mucus from the nostrils, but they seldom do serious injury. The (E. bovis lays its eggs on the backs and sides of oxen and cows. The larva3, hatched by the heat, penetrate the skin, and by increase of size form tumors as large as pigeon's eggs.

They live upon the pus their presence produces. After a time they make a larger aperture, and, creeping out, seek a proper place in which to become chrysalids. The fly when discovered creates a great panic among cattle, and often drives them frantic to the nearest pool. Birds, too, are nearly all infested by lice, each species generally supporting its own species of parasite, and sometimes more, which lives upon the feathers and blood of its host. - Insects also, fortunately, are made a dwelling place by other insects, and thus their rapid growth and the consequent destruction of vegetation held in check. Ichneumon is the name given to these unnatural parasites. They are small flies with slender bodies, and there are many species known, probably as many as of caterpillars and moths. The female deposits her eggs in the larvae, pupa?, or eggs of other insects and spiders. When she has found her proper host, a caterpillar for instance, she ' seizes it, and deposits her egg in the skin behind the head. The larva, soon emerging from the egg, eats its way along within the caterpillar, avoiding those parts essential to life, and by the time the latter has become a chrysalis the former is nearly mature.

It lies quiet for a time to undergo metamorphosis, and awaking once again a perfect animal, bores its way out from the cocoon of its murdered host, and flies forth in quest of fresh victims. Thus it is that nature keeps in check its most destructive creatures by means so insignificant and unseen. - See Baird, "British Entomos-traca" (Ray society, London, 1850); Brant and Katzeburg, Medicinisclie Zoologie (1833); Burmeister, "Manual of Entomology," translated by Shuckard (London, 1836), and Genera Inscctorum (Berlin, 1833-46); Newman, "History of Insects" (London, 1839); Westwood on "Insects" (2 vols., London, 1839), and bibliography therein contained; Denny, Mono-graphia Anoplurorum Britannioe (London, 1842); Siebold, "Anatomy of Invertebrata," translated by Burnett (Boston, 1854); Wedl's Grundzuge der pathologischen Anatomie (Vienna, 1854); and Kuchenmeister, "Manual of Parasites," translated for the Sydenham society (London, 1857).

Skin Mite (Demodex folliculorum).

Bird Mite (Dermanyssus avium).

Red Mite (Leptus autumnalis).

Head Louse (Pediculus capitis).

Phthirius pubis.

Bedbug (Cimex lectularius).

Flea (Pulex irritans).

1. OEstrus Bovis. 2. Larva. 3. Chrysalis.