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Page:Popular Science Monthly Volume 19.djvu/398

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384
THE POPULAR SCIENCE MONTHLY.

is limited to one sex alone, and that the free-winged males of stylops are as highly organized as most of their neighbor insects.

The class of the spiders (Arachnida) offers collective examples of degeneration and retrogression, which show how large numbers of animals may acquire lower characters, contrasting with the higher phases to which other members of their class have attained. The mites and ticks have unquestionably originated from the same root-stock as the spiders and scorpions. The development of the two groups proves this much. But, while the latter animals have advanced to a high complexity of organization, the mites and ticks have degenerated into parasitic forms—or at least exemplify beings which, first attaining a respectable rank in their own series, have certainly not advanced upon that rank. Many of the mites, however, exhibit well-marked degeneration. Only on the hypothesis of sweeping retrogression can we account for the singular and anomalous condition in which a certain harmless mite, named Demodex folliculorum (Fig. 14), spends its existence. This mite inhabits the sacs or follicles of the human skin at the sides of the nose. It is a minute, worm-like animal, possessing eight degenerate rudiments of legs, and a thoroughly rudimentary structure in other respects. Here parasitism has denuded the

PSM V19 D398 Demodex and linguatulina.jpg
Fig. 14.—Demodex (magnified). Fig. 15.—Linguatulina.

animal of wellnigh every attribute of its Arachnidan character, and has left it in a condition analogous in many respects to sacculina itself. Of the equally curious Linguatulina (Fig. 15), inhabiting the "frontal sinuses" or forehead spaces of dogs, wolves, horses, and sheep, the same remark holds good. The body here is thoroughly worm-like in shape (b, c), and a digestive and nervous system are to be enumerated among the possessions of the organism. But not even the rudiments of legs are to be perceived, although the mouth bears certain apologies for the appendages proper to that region in the mite and spider class. Yet the young linguatulina (a) exactly resembles the early form of the