152 MANUAL OF ZOOLOGY. 



a distinct anus. They also possess a system of canals, in some 

 cases contractile, which open externally near the anterior part 

 of the body, and are probably homologous with the water- 

 vascular system of the Toeniada and Trematoda. (Huxley.) The 

 sexes are distinct, and the males are usually less frequently 

 met with, and of smaller size than the females. The nervous 

 system is mostly well developed, and is in the form of a gan- 

 glionic ring, surrounding the oesophagus, and sending filaments 

 backwards. 



As before said, most of the Nematoda are internal parasites, 

 inhabiting the alimentary canal, the pulmonary tabes, or the 

 areolar tissue, in man and in many other vertebrate animals ; 

 but a large section of the order are of a permanently free habit 

 of existence. 



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 directly 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 fe- 

 males are much bigger than the males. 



The Trichina spircdis is a singular Nematoid, which gives rise 

 to a painful and very generally fatal train of symptoms, some- 

 what resembling rheumatic fever, and known as Trichiniasis. 

 The Tricliina 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 in a little capsule or cyst. In this con- 

 dition 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 a numerous progeny, which they produce 



