//. NEMATHELMINTHES : NEMATODA. 



299 



The mouth connects with the muscular oesophagus, which, for 

 sucking purposes, is expanded posteriorly to a pharyngeal bulb and 

 is lined throughout with a cuticle. From this point to the anus 

 the stomach-intestine is usually uniform (fig. 263). The oesophagus 

 is surrounded by a nervous ring which sends forward and back a 



FIG. 263. FIG. 264. 



FIG. 263. Structure of young female Ascaris (based on a drawing by Leuckart). 

 d, intestine; o, ovary; s, lateral line; v, ventral line: va, vagina. 



FIG. 264. Diagram of nervous system of a nematode. (After Butscbli.) c, commis- 

 sures; d, dorsal nerve; z, infraoesophageal, s, supraoesophageal part of nerve 

 ring ; v, ventral nerve. 



large number of nerves, those in the mid-dorsal and ventral lines 

 being strongest. At points on these nerves are collections of 

 ganglion cells, but a formation of ganglia, as in the annelids, does 

 not occur (fig. 264). 



The sexual organs of these rarely hermaphroditic forms are 

 very simple. Males and females are easily distinguished not only 

 by the copulatory organs, but by the openings of the genital ducts. 

 These, in the male (fig. 265), are in the end of the alimentary 

 canal, which hence is a cloaca. In the female (fig. 263) there is a 

 special genital opening on the ventral surface between mouth and 

 anus, the position varying with the species. In general the struc- 



