162 Nemathelminthes. 



Horse. In the yovuigest stage it is free and is probably swallowed by the Horse 

 with di-inking-water ; it lives at first in certain arteries (especially in the anterior 

 mesenteric artery), which suffer, in consequence, a pathological change (worm- 

 anem-ism). Later it passes into the gut, and attains sexual matui-ity. How the 

 wanderings to and from the artery ocoiu- is not yet known, Neitlier the presence 

 of the worm in the ai-tery nor in the gut seems to affect the health of the Horse 

 directly ; biit a clot from the aneui-ism may occasion a stoppage in the intestinal 

 vessel, and thei-eby a dangerous, often fatal, illness. Other Strongylidte live in 

 various domestic animals, among them the dangerous S. filaria in the lung of 

 the Sheep. 



4. Trichocephalus dispar, very frequent in the large intestine, especially the 

 CEECVim of Man, rare in England ; the front part of the body is drawn out to a 

 long, thin thi-ead, which bores into the mticous membrane of the gvit ; may attain a 

 length of 5 o/m. The embryo develops within the egg-shell in damp places or in 

 water, and is taken, still enclosed by the shell, into the digestive tract of the host, 

 where it hatches and undergoes fui'ther development. 



5. The Trichina (Trichina spiralis). The body of the sexually mature 

 animal, the so-called Intestinal-trichina, is very thin; the female, 3 to 

 3'5 m/m., the male, 15 m/m., long; the female aperture lies far forward; the 

 hind end of the male, with two cones, cloaca eversible, sei'ving as a copulatoiy 

 organ ; spicula wanting. In the matm-e state in the small intestine of Man and 

 other Mammals, especially in the Pig and the Rat. The Intestinal-trichina 

 produces a large number of microscopic larvss whilst within the gut of the host 

 (each female at least about 1500), which at once bore through the intestinal wall 

 into the body-cavity of the same host, and thence migrate into the muscles, 

 where each enters a muscle-fibre, causing it to swell up. The outer pai-t of the 

 swollen muscle-fibre hardens into a citron-shaped capsule round the young 

 Trichina, which, meantime, has grown considerably (to I m/m. long), and now 

 lies spirally coiled in the pulpy mass filling the capsule ; the capsule, after 

 some months, is infiltrated with calcareous salts, and becomes hard and opaque. 

 If an animal, which contains such encysted Muscle-trichina, is eaten by 

 another (in which the Trichina can live), the capsules are dissolved in the stomach, 

 the TrichinsB are freed, pass to the intestine, and attain sexual maturity in the 

 course of a few days. They then copulate, and within a week after their 

 entrance into a new host, the female intestinal form produces the first embi-yos. 

 The female usually lives in the intestine only five or six weeks, and then dies ; 

 the adult male lives for a stiU shorter time. As Muscle-trichinse, they can, 

 however, live even several years; old forms often become calcified, and then 

 die. Man is infected by partaking of raw pork ; the Pig, by eating a Rat ; the 

 latter, probably, by devoui-ing the sweepings of the slaughter-house, or a dead 

 comrade. Trichinosis is really caused by the wanderings of the young Trichina, 

 and by its first sojourn in the muscle ;_ when the wandering is over and the 

 Trichina is encysted, the symptoms of disease cease, but recovery is often very 

 gradual, and many oases terminate fatally. 



6. The Threadworms (genus, Filaria, etc.) are very elongate animals, 

 living as a rule, in parts of the host other than the gut, chiefly in connective 

 tissue. Amongst them ai*e : 



(a) The Guinea- or Medina-worm {F. [Bracwnculus'] medvnensis) living 

 in the connective tissue under the skin or between the muscles of man ; only, 

 however, in warm regions of the Old World. The female alone is known ; it 

 attains a length of 80 o/m. In the adult, the digestive tract is atrophied, the anus 

 is absent, food is taken by absorption through the body-wall. The gi-eater piirt 

 of the body-cavity is occupied by the enormous mouthless oviduct, in which there 

 are several million embryos. The irritation produced by the parasite, causes 

 small abscesses through which the mature worms make their escape. The larva 



