annuloida: scolecida. 



177 



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

 cases contractile, which open externally near the anterior part 

 of the body, on the ventral surface, or by lateral pores, and are 

 probably homologous with the water-vascular system of the 

 TcBuiada and Trematoda. 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 ganglionic ring, surround- 

 ing the oesophagus, and sending filaments backwards. 



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

 inhabiting the alimentary canal, the pulmonary tubes, 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 

 hunbricoides, the little Oxyuris, the 

 Trichina, and the Guinea-worm. 



The Ascaris lumbricoides, or com- 

 mon Round-worm, inhabits the intes- 

 tine of man, often attaining a length 

 of several inches. The ova are pro- 

 bably 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, 



"Small Thread -worm," is a 



worm, which inhabits 



especially of children. 



ganous 

 rectum, 

 the 



IS 



or 

 gre- 

 the 

 It 

 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 re- 

 sembling rheumatic fever, and known as Trichiniasis. The 



Fig. 52. — KematcMa. A AngU' 

 illula aceii; B Dorylaimui 

 stagnalis. Magnified. 



