210 PAEASITES OF MAN. 



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 httle ttreadworm, (Oxyurfs.j partly 

 fi-om the circumstance that the large round worm (Axcaris) bears a 

 marked resemblance to the common lob-worm of onr gardens, 

 (Lumhricus,) and partly, or perhaps chiefly, because the spu-al flesh- 

 worm (Trichina) plays an important n'de in the pi'oductiou 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 

 more absurd than the popular notion that nematoid entozoa, especially 

 threadworms, (O.rijurides,) ai-ise or make their appearance in consequence 

 of an impoverished state or cachexia of the body of the host. This ridicu- 

 lous conception, which is as old as the hills, is ever and anon re-asserted 

 with all the pride and confidence which should only be displayed when any 

 real and valuable discoveiy 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 genera- 

 tionists always pointed, triumphantly as they supposed, to the mode of 

 origination of the entozoa. The truth is, neither threadwonns nor 

 helminths of any other kind arise from diseased conditions. They often 

 produce constitutional disturbance in their victims, this bad effect being 

 misinterpreted as a cause of the appearance of the entozoa themselves. 

 A healthy person is just as Ukely — nay, he is even more likely to entertain 

 parasites than a feeble person. True, the strong host may suffer com- 

 paratively little, whilst the weak host succumbs to his guests. The host 

 is the entozoon's native territory. What our native island is to us, our 

 bodies ai'c to parasites. To attack, to invade, to infest, is their legitimate 

 pi-erogative ; and for this end it must be admitted that theii- 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 too often operate to obscure the mental vision, and 

 thus prevent the adoption of measures calculated to check not a few of 

 the many evils to which our common flesh is heir. 



Nematoda. 

 25. — Trichina si)iralis, Owen. 



SjTionymy. — Psejidalius trichina, Davaine. 



Larvffi. — Commonly spoken of as muBcle-trichinae, capsuled or 

 encysted trichina), and fleshworms. 



Intermediate Host. — AU warm-blooded animals, especially mam- 

 mals, and of these the hog and rat more especially. Man him- 

 self may become au intermediate beai'cr. 



