Parasites belonging to this class occur both in man and animals. In one form the larva inhabits the internal parts, while the adult is external.

Acarus Scabiei, Or Sarcoptes Hominis

This has an oval body just large enough to be visible to the naked eye, the female being one fiftieth of an inch in length, and the male about half that size. The anterior part has a head and four limbs, each of which has a sucker at its extremity. There are also four posterior limbs, all of which in the female have pointed extremities, but in the male two of them have suckers. The female burrows in the epidermis, forming tunnels, in which, as it proceeds, it deposits its eggs; it is usually to be found at the deepest end of the tunnel and the eggs at intervals. The eggs develop, and, as the epidermis desquamates, they come to the surface by degrees, the young being born usually as they reach the surface. The irritation of the animal in the epidermis gives rise to a slight inflammation, causing the formation of a papule. Usually there is great itching, and the scratching leads to further eruptions, especially in predisposed persons.

Acarus Folliculorum (Demodex Folliculorum)

This is an elongated animal about 1/125 of an inch in length, and provided anteriorly with four pairs of short feet. It is found in the sebaceous follicles, especially of the external meatus of the ear and neighbourhood of the nose. It seems to produce no special irritation.

Pentastoma Denticulatum S. Tsenioides

The larval form is very common in the rabbit, and has been not infrequently observed in man. It occurs usually in the liver, but has been observed also in the spleen, lungs, kidneys, and wall of intestine. It is a small animal about a fifth of a inch in length and a fifteenth in breadth. It presents about 90 segments, in each of which are stomata. The mouth possesses four hooks which can be withdrawn into chitinous sheaths. The larva in the liver surrounds itself with a capsule and forms a nodule about the size of a pea. In man they are mostly found dead, and the condition observed is that of a hard nodule surrounded by a fibrous capsule, inside which are the calcified remains of the animal, of which only the hooks may be recognizable.

The larva found in the liver and elsewhere is usually designated the Pentastoma denticulatum, while the adult is called the P. tsenioides. The connection between the two was demonstrated by Leuckart. The adult is found chiefly in the nares of dogs, and also of some other animals, and on one occasion of man (Laudon). The adult is like the larva in form but much larger, the female measuring three inches, and the male about one. The mouth is devoid of hooks. The ova passing from the nostrils of the dog on to the grass are supposed to be taken into the stomachs of hares and rabbits, and to pass thence to the liver.

Many forms of arachnida occur in animals.

Leptus Autumnalis (Harvest Bug)

This is a small red animal, just visible to the naked eye. It is not a necessary parasite, but in some districts it invades the legs and burrows into the skin, thus causing excessive itching. Two other forms of leptus are described as occurring in America (Duhring).