The nematoid group of parasites, next to be considered, are probably better known than any other helminths. This arises partly on account of the excessive frequency of the little threadworm, (Oxyuris,) partly from the circumstance that the large round worm (Ascaris) bears a marked resemblance to the common lob-worm of our gardens, (Lumbricus,) and partly, or perhaps chiefly, because the spiral flesh-worm (Trichina) plays an important rôle in the production of epidemic disease (Trichinosis.) Endless mistakes have arisen from the error of confounding parasitic roundworms with earthworms, The mischievous character of unscientific or inexact knowledge may be illustrated by the fact that I have known nervous persons so seriously alarmed at the appearance of lumbricoid entozoa that they have regarded their presence as an omen of approaching dissolution. I have even known a spurious nematoid to be dreaded as "the worm that dieth not." It is very important that correct views should be entertained respecting the nature and sources of the various members of this group of parasites. Nothing is move absurd than the popular notion that nematoid entozoa, especially threadworms, (Oxyurides,) arise or make their appearance in consequence of an impoverished state or cachexia of the body of the host. This ridiculous conception, which is as old as the hills, is ever and anon re-asserted with all the pride and confidence which should only he displayed when any real and valuable discovery has to be announced, The notion, as it now stands, is a feeble remnant of the theory of equivocal generation, For the establishment of the truth of this theory the spontaneous generationists always pointed, triumphantly as they supposed, to the mode of origination of the entozoa. The truth is, neither threadworms nor heliminths of any other kind arise from diseased conditions. They often produce constitutional disturbance in their victims, this bad effect being misinterpreted as» cause of the appearance of the entozoa themselves. A healthy person is just as likely—nay, he is even more likely to entertain parasites than a feeble person, True, the strong host may suffer comparatively little, whilst the weak host succumbs to his guests. The host is the entozoan's native territory, What our native island is to us, our bodies are to parasites. To attack, to invade, to infest, is their legitimate prerogative; and for this end it must be admitted that their organisation is admirably adapted. To be sure, it is equally our prerogative to refuse the would-be guests admission, but any method of resistance likely to prove effectual must be based upon scientific conclusions resulting for the most part from experimental research: Ancient dogmas and pre-conceived opinions to often operate to obscure the mental vision, and thus prevent the adoption of measures calculated to cheek not a few of the many evils to which our common flesh is heir.
Nematoda.
25.—Trichina spiralis, Owen.
