ANNULOSA: NEMATODA. 



245 



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. 



Fig. 120. Morphology of Nematoda. a Ascaris lumbricoides, male, reduced in size ; 

 b Hinder extremity of the same, with the reproductive spicula, enlarged ; c Head of 

 the same enlarged, showing the tubercles round the mouth ; d Ovum of the same, 

 highly magnified, with the fully-developed worm in its interior; e Male of Oxyuris 

 vermicnlaris, five times the natural size ; f Female of the same, similarly enlarged ; 

 g Male of the same, highly magnified ; h Embryo of Dracunculus, magnified 500 

 diameters ; i Embryos of the same, magnified 60 diameters ; / A single Trichina, 

 encapsuled in the muscles, highly magnified. (Chiefly after Leuckart, Spencer 

 Cobbold, and Bastian.) 



Amongst the more important members of the parasitic 

 section of the Nematoda may be mentioned the Ascaris lum- 

 bricoides, the Oxyuris vermicularis, the Trichocephalus dispar, 

 the Sderostoma duodenale, the Dracunculus medinensis, and the 

 Trichina spiralis. 



The Ascaris lumbricoides, or common Round-worm, inhabits the intestine 

 of man, and sometimes of other mammals, especially the pig, often attain- 

 ing a length of several inches. The ova are expelled with the faeces, and 

 the embryo is developed within the ovum prior to its rupture, but not till 

 after the lapse of several months (fig. 120, d). 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 its host. The body of the adult 



