298 ZOOLOGY 



SECT. 



in a chitinoid egg-shell, and are passed out of the body of the host 

 with its faeces. The results of experiments render it probable that 

 infection is direct, without intermediate host, the embryo-containing 

 eggs being taken, in water, or in soil accidentally swallowed, into 

 the intestine of a new human host, in which the embryos, escaping 

 from the eggs, become mature. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Nematoda are Nemathelminthes having a cylindrical body 

 of great length in proportion to its diameter, and pointed at both 

 ends. The body-wall consists of a tough external cuticle, an 

 ectoderm in the form of a syncytium or protoplasmic layer con- 

 taining nuclei and rarely exhibiting cell-structure, and a single 

 layer of longitudinal muscular fibres which are interrupted along 

 one or more (dorsal, ventral, and lateral) lines. The body-wall 

 encloses a body-cavity containing a clear fluid, and more or less 

 encroached upon by processes of the muscle-cells or other meso- 

 dermal tissues. The enteric canal is straight, and consists of 

 pharynx, intestine, and rectum : the pharynx is a stomodseum. 

 The mouth is anterior and terminal, the anus ventral and situated 

 a short distance from the posterior end. Excretory canals, running 

 in the lateral lines, are usually present. The nervous system con- 

 sists of a pharyngeal ring containing nerve-cells and giving off 

 nerves forwards and backwards : of those passing backwards there 

 is either a single ventral-cord, or there are two chief cords, respec- 

 tively dorsal and ventral, of considerable size and extending to the 

 posterior end of the body. The Nematoda are in nearly all cases 

 dioecious : eggs are produced in immense numbers, and are impreg- 

 nated 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, 

 etc. ; 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 sub-cuticle is a nucleated protoplasmic 

 layer without cell-outlines. 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 consist of long, more or less con- 

 voluted cords. This order includes nearly the whole of the free- 

 living Nematodes as well as the large majority of parasitic forms. 



