ANNULOIDA I SCOLECIDA. 



195 



ORDER III. NEMATODA (or Nematoidea). The Nematoda 

 " Thread-worms" or " Round- worms " are of an elongated 

 and cylindrical shape ; and are often, though by no means 

 always, parasitic in the interior of other animals. They possess 

 a distinct mouth and an alimentary canal which is freely sus- 

 pended in an abdominal cavity, and terminates posteriorly in a 

 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 

 Tceniada 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, surrounding the oesophagus, 

 and sending filaments backwards. 



As before said, most of the Ne- 

 matoda are internal parasites, in- 

 habiting 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 

 lumbricoides, 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 vermmilaris, or 

 "Small Thread-worm," is a grega- 

 rious worm, which inhabits the rectum, especially of children. 

 It is the smallest of the intestinal worms of man, its average 



Fig. 72. Nematoda. A, Angn- 

 illula aceti ; B, Dorylaimus 

 stagnates. Magnified. 



