Page:The American Cyclopædia (1879) Volume VI.djvu/706

694 EPIZOA important of the external parasites of the ani- mal 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 loco- motion, 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 suste- nance and habitation, and that, more inde- pendent 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 in- sects, which cannot live beneath the water. These are subdivided into the lerneadce 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 ; Linnasus 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 con- sistency, 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 pos- teriorly, 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 despe- ration 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, flat- tened 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 ani- mals. Particular species generally infest par- ticular 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 dis- tance. 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 abdo- men are blended in one oval mass. In their im- mature 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. : linguatula 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 sub- ject investigated, by Henle and Simon at the same time. The head of this microscopic para- site 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 diffi- culty, and are tipped according to some au- Skin Mite (Demodex folliculorum). thorities with three claws, to others with but one. Several forms are met with owing to dif- ference 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 an-