THE ACARIDM. 



185 



In their habits, as in their structure, they are most various. Thus a considerable number, of 

 which the common CHEESE MITE (Tyroglyplms domesticus) may be taken as the type, live either in dry 

 or decaying animal and vegetable materials, or upon the roots of plants, such 

 as Liliacete (between the scales), potatoes, dahlias, <fcc., or in Agarics. Cheese, 

 flour, and sugar are also favourite substances with these little creatures. The 

 members of the genus Hypopus and its allies, distinguished by having the two 

 anterior pairs of feet fairly or well developed, while the two hinder pairs are 

 small and concealed beneath the body, are found externally parasitic upon a 

 variety of insects, and also occasionally upon vertebrate animals. Hypoderas, 

 on the contrary, the species of which have a more or less elongated body, 

 with two pairs of legs issuing from close to the anterior end, and two pairs 

 from much farther back, includes internal parasites, some of them living under 

 the skin, or in the muscles of various vertebrate animals, and others in their 

 bronchial tubes and lungs, or in the air-cells which exist under the skin in TYROGLYPHUS DOMESTICUS. 

 many birds. Occasionally these minute parasites occur in immense numbers. 



A very considerable number of species, belonging to the genus Sarcoptes and its allies, are parasitic in 

 the skins of various vertebrates, upon which they cause the disease commonly known in man as " the 

 itch." They are generally of a broadly ovate or rounded figure, with the skin more or less distinctly 

 striated across, and furnished with the usual four pairs of legs, placed half towards the front and half 

 towards the posterior part of the body, the legs generally having sevei'al bristles, and terminating in a 

 slender tarsal part with a sucker at the end. The chelicerse are nipper-like, and it is by the agency of 

 these that the parasites burrow beneath the epidermis of the animals they infest. The species attack- 

 ing human beings in Europe generally is the Sarcoptes scabiei ; but in Iceland and the northern part 

 of Europe another form is so common that in some localities scarcely any of the inhabitants seem 

 able to escape from it, and this produces a much more formidable complaint than the common Itch- 

 mites. When the disease is allowed to proceed unchecked the parts of the body attacked by it 

 become coated with a sort of crust, which is said to consist of the dead Sarcoptes, massed together by 

 some viscid fluid, thus often simulating elephantiasis. Allied forms attack the fox, dog, 

 cat, goat, pig, rabbit, fowl, horse, ox, and other quadrupeds and birds. Besides, there 

 are a good many louse-like Mites, related to the pi-eceding, which are found as 

 surface parasites upon mice, bats, and birds, in various countries. These sometimes have 

 the tarsi terminated by curved claws, which assist them in clinging to the hairs of 

 their victims. Most of them have sucking mouths, but some appear to be organised for 

 biting. In connection with these parasites we may mention the curious species Demodex 

 folliculorum, a microscopic, worm-like creature, closely ringed throughout, furnished 

 towards the anterior end with four pairs of very short limbs, each of which terminates 

 in two claws. The mouth is suctorial. The larva has only three pairs of legs. This 

 singular little parasite occurs pretty frequently in the hair-follicles and sebaceous glands 

 of the skin in man, where it often gives rise to pimples, from which it may be 

 squeezed by a careful application of the nails. It is, no doubt, the " maggot in cheese- 

 monger's nose " commemorated by Butler in " Hudibras," though we are not aware that 

 it bestows its visits especially upon any particular class of tradesmen. A very considerable 

 number of species referred to this family, and chiefly to the genus Phytoptus, attack the 

 living tissues of plants, co-operating with the Aphides in their attacks upon the leaves and 

 other growing parts, and, like them, often causing the formation of peculiar gall-like 

 deformities and excrescences. The Mites, like the Aphides, generally attack the under side of the 

 leaves, and cause them to grow up into hollow excrescences of various forms, in the interior of which 

 the Mites live. Other allied species attack the buds of trees and materially injure their growth. 



ORDER IV. TARDIGRADA. 



This is a small group of microscopic creatures commonly known as Bear- or Sloth-animalcules, 

 found in moss and in wet places, and displaying some peculiarities which have for a long time 

 rendered them exceedingly interesting to microscopists. They have a longer or shorter oblong- 

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DEMODEX 

 TOLLICCLOKUM 



