304 ZOOLOGY sect. 



of considerable size and extending to the posterior end of the body. 

 The Nematoda are in nearly all cases dicecious : eggs are pro- 

 duced in immense numbers, and are impi-egnated within the body 

 of the female. The sperms are non-motile, or perform amoeboid 

 movements only after entering the female organs. Cilia are 

 wholly absent. 



A large proportion of Nematoda are free-living, spending their 

 whole life in fresh or salt water, damp earth, decaying matter, 

 &c. ; the remainder are parasitic during the whole or a part of 

 life. 



The class is divided into two orders. 



Order 1. — Nematoidea. 



Nematoda in which the body-cavity is not lined by epithelium, 

 but is bounded directly by the body-muscles. Two chief nerve- 

 cords are given off backwards from the pharyngeal ring and 

 lie in the dorsal and ventral lines. There are two excretory 

 canals lying in the lateral lines and opening anteriorly and 

 ventrally. The gonads are continuous with their ducts, and con- 

 sist of long, more or less convoluted cords. This order include.s 

 the whole of the free-living Nematodes as well as the large 

 majority of parasitic forms. 



Order 2. — Nematomorpha. 



Nematoda in which the body-cavity is lined by a distinct epithe- 

 lium. The pharyngeal nerve-ring sends off a single large ventral 

 nerve-cord well supplied with nerve-cells. The gonads, or at least 

 the ovaries, are arranged metamerically, and the reproductive pro- 

 ducts are discharged into the body-cavity and pass thence into the 

 gonoducts. This order includes a small number of greatly 

 elongated, thread-like worms (species of the genus Gordiics), which 

 are parasitic in the asexual, free-living in the sexual stage ; and 

 also the genus Nechirus, which has only been found swimming 

 in the sea. 



Systematic Position of the Example. 



Ascaris lumbricoides is one of many species of the genus Ascaris, 

 and belongs to the family Ascaridw of the order Nematoidea. 



The absence of an epithelial Uning to the body-cavity, and the 

 presence of elongated gonads continuous with their ducts, indicate 

 its position as one of the Nematoidea. Among the numerous 

 families constitutmg this order, the Ascaridse are distinguished by 

 the possession of three lips furnished with papillae, and by the 

 body of the male being curved ventrally and being provided 

 with penial setae. Ascaris is distinguished from the other 



