MANUAL OF ZOOLOGY. 



The most familiar examples of the parasitic Nematoda are 

 the Ascaris lumbricoides, the little Oxyuris, the Trichina, and 

 the Guinea-worm. 



The Ascaris lumbricoides, or common Round-worm, inhabits 

 the intestine of man, often attaining a length of several inches. 

 The ova are probably expelled with the faeces, and the embryo 

 is developed within the ovum prior to its rupture, When 

 fully formed, the embryo is about 

 one-hundredth of an inch in length, 

 and its development is not exactly- 

 known, though it appears to be di- 

 rectly transferred from river or pond 

 water to the alimentary canal of some 

 vertebrate animal. 



The Oxyuris vermicularis, or 

 "Small Thread -worm," is a gre- 

 garious 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 Trichina spiralis is a singular 

 Nematoid, which gives rise to a pain- 

 ful and very generally fatal train of 

 symptoms, somewhat resembling 

 rheumatic fever, and known as 

 Trichiniasis. The Trichina is known 

 in two different conditions, sexually 

 immature or mature. In its sexually 

 immature condition it inhabits the 

 muscles, usually of the pig, in vast 

 numbers, each worm being coiled up 



Fig. 50. Nematoda. A Angu- i n a little Capsule Or CVSt In this 



^L^ki^ aimus condition the worm is incapable of 

 further development, and may re- 

 main, 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 

 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 



