: 



PHYLUM NEMATHELMINTHES 



281 



the precise manner in which the worm gains access to the human 

 intestine. It is possible that the eggs containing developing 

 embryos, or the embryos themselves, after liberation from the egg- 

 shell, may be taken in by drinking, without previous filtering, water 

 into which fecal matter has been discharged. On the other 

 hand it is quite possible that there may be an intermediate host 



derepthm, 



FIG. 227. Ascaris lumbricoides, posterior extremity of male, dissected, an. arms ; 

 cu. cuticle ; der. epthm. epidermis ; m. muscular layer ; p. n. peuial seta ; s. sacs containing 

 penial seta ; ts. testis ; rs. MIL vesicula seminalis. 



such as we have met with in the Flukes and Tape-worms, and shall 

 also find to occur in several members of the class now under 

 discussion. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Nematoda are Nematelminthes haying 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 the latter a single ventral 

 nerve-cord, or two cords, respectively dorsal and ventral, are of 

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

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



