246 MANUAL OF ZOOLOGY. 



(fig.' 1 20, a) is cylindrical, attenuated at both extremities. At the anterior 

 extremity is a sub-triangular mouth, surrounded by three tubercles (fig. 

 120, c}. The anus is situated posteriorly. The females are larger than 

 the'males, and are much more numerous. These parasites are much more 

 common in children than in adults, and they often occur in considerable 

 numbers. They are usually found in the small intestine, though they some- 

 times wander into other situations. 



The Oxyuris vermicularis, or "small Thread- worm" (fig. 120, e, f, g), 

 is a gregarious worm, which inhabits the rectum, especially of children. 

 It is the smallest of the intestinal worms of man, its average length not 

 being more than a quarter of an inch, but the females are much bigger 

 than the males. 



The Trichocephalus dispar inhabits the caecum of man. It is from one 

 and a half to two inches in length, and the anterior two-thirds of the body 

 is extremely attenuated and thread-like. 



The Sclerostoma duodenale inhabits the small intestine in the human sub- 

 ject, and is far from uncommon in Italy and in Egypt. It varies in size from 

 one-third of an inch to half an inch, the females being the largest ; and the 

 symptoms to which it gives rise are often of a serious character. 



The Trichina spiralis is a singular Nematoid, which gives rise to a pain- 

 ful and not uncommonly fatal train of symptoms, somewhat resembling rheu- 

 matic fever, and known as Trichiniasis. The Trichina is known in two 

 different conditions, sexually immature or mature. In its sexually imma- 

 ture condition it inhabits the muscles, usually of the pig (the rat, however, 

 being apparently its true host), in vast numbers, each worm (fig. 120, /) 

 being coiled up in a little capsule or cyst. In this condition the worm is 

 incapable of further development, and may remain, apparently for an in- 

 definite period, without change, and without seeming to produce any in- 

 jurious results to the animal affected. If, however, a portion of trichina- 

 tous muscle be eaten by a warm-blooded vertebrate, and so introduced 

 into the alimentary canal, an immediate development of young Trichina is 

 the result. The immature worms escape from their enveloping cysts, grow 

 larger, develop sexual organs, and give birth to a numerous progeny, which 

 they produce viviparously. The young Trichina thus produced perforate 

 the walls of the alimentary canal, and, after working their way amongst 

 the muscles, become encysted. If the animal in which these changes go 

 on has sufficient vitality to bear up under the severe symptoms which are 

 produced by the migration of the Trichina, he is now safe ; since they 

 cannot become sexually mature, or develop themselves further, until again 

 transferred to the alimentary canal of some other animal. 



The Guinea-worm (Dracunculus or Filaria medinensis) is a Nematode 

 worm, which inhabits, during one stage of its existence, the cellular tissue 

 of. the human body, generally attacking the legs, and often attaining a 

 length of several feet. All known specimens of this parasite are impreg- 

 nated females, containing a large number of young. The worm remains 

 embedded in the body, in a more or less quiescent condition, for a year or 

 more, at the end of which time it seeks the surface, in order to get rid of 

 its young. No external aperture to the genital organs has hitherto been 

 proved to exist, and it seems possible that the young are produced within 

 the body of the parent by a process of internal gemmation. The young 

 Filaria (fig. 120, h and z) consists of a vermiform body, terminating in a 

 hair-like tail ; and when set free from the parent, its further development 

 probably takes place in water, when it is believed to be converted into one 

 of the "Tank- worms" so common in India. In this condition, it is pos- 

 sible, as some believe, that sexual organs are developed, and that the 

 females are impregnated. The young worm is believed to pass the inter- 



