196 MANUAL OF ZOOLOGY. 



length not being more than a quarter of an inch, but the 

 females are much bigger than the males. 



The Trichina spiralis is a singular Nematoid, which gives rise 

 to a painful and very generally fatal train of symptoms, somewhat 

 resembling rheumatic fever, and known as Trichiniasis. The 

 Trichina is known in two different conditions, sexually imma- 

 ture or mature. In its sexually immature condition it inhabits 

 the muscles, usually of the pig, in vast numbers, each worm 

 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 indefinite period, without change, 

 and without seeming to produce any injurious results to the 

 animal affected. If, however, a portion of trichinatous muscle 

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

 the alimentary canal, an immediate development of young 

 Trichinae is the result. The immature worms escape from 

 their enveloping cysts, grow larger, develop sexual organs, 

 and give birth to 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 Zilaria 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 impregnated females, 

 containing a large number of young. The worm remains 

 imbedded 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 

 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 con- 

 verted into one of the " Tank-worms " so common in India. 

 In this condition, it is possible, as some believe, that sexual 

 organs are developed, and that the females are impregnated. 

 The worm is believed to gain access to the body of bathers, 



