vi PHYLUM NEMATHELMINTHES 303 



Development. — The eggs arc produced in immense numbers — 

 at the rate, it has been reckoned, of about 15,000 a day. They 

 are fertilised in the upper part of the uterus, each becoming 

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

 the host with its faeces. Segmentation is "complete, but the 

 details of development are not known in this species. The results 

 of experiments render it probable that infection is direct, without 

 intermediate host, the embryo-containing eggs being taken, in 



tnt 



der.e/jthm. 



1 



Fio. 242. — Asearis lumbricoides, posterior extremity of male, dissected, an, anus ; 

 cu. cuticle ; der. eptltm. epidermis ; m. muscular layer ; p. s. penial seta ; s. sac containing 

 penial seta ; ts. testis ; vs. sem. vesicula seminalis. 



water, or in soil accidentally swallowed, into the intestine of a new 

 human host, in which the embryos, escaping from the eggs, become 

 mature. - S 



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 stomodgeum. 

 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 these there is either a single 

 ventral-cord, or there are two cords, respectively dorsal and ventral, 



