24 ILLINOIS BIOLOGICAL MONOGRAPHS [128 



to force the food down upon the teeth and rasps by which means it is 

 torn apart and made ready for swallowing. 



Another type of armature is that found in Synonchium obtusum Cobb. 

 Here what is apparently the pharynx is highly muscular and forms a large 

 muscular bulb. The mouth is really the large flaring, triangular opening 

 to the pharynx located in a slightly shallow depression formed by the six 

 double, amalgamated, flaring lips. The sectors of the pharynx are equal 

 and each is armed in its mid line by a mandible mounted by three in- 

 pointing teeth and flanked on each side by a small tooth (Fig. 16). When 

 the pharyngeal bulb contracts, the mandibles are approximated, drawn 

 inward and downward while the lips are slightly raised, partly covering 

 the mandibles. Another nematode, Xyala slricta Cobb, has three similar 

 but less elaborate mandibles. Still another example of the open flaring and 

 armed pharynx may be found in Gammanema ferox Cobb. Here the base 

 of the pharynx is armed with three ribs, each bearing an inward pointing 

 onchium. Jointed mandibles (Fig. 7) are present in Scaptrella cincta 

 Cobb, and inpointed ones are again present in Cheironchus vorax Cobb and 

 in Selachinema. 



There are forms which have greatly developed dorsal onchi which 

 have become in many cases much elongated (Anaxonchia) and well buried 

 in the esophageal musculature. These spear-shaped onchi are movable and 

 perhaps they form, as Cobb has suggested, the transition from the pharynx 

 armed with onchi to the spear-bearing pharynx. There is still another 

 spear-bearing group in which the so-called stylet is hollow, allowing the 

 fluids to be drawn through it by suction created in the esophagus. Dorylai- 

 mus and Discolaimus are good examples of this construction. Some species 

 indicate by the structure of their stylets that these organs may have 

 arisen by the partial fusion of three onchi and a few of them distinctly show 

 construction from three portions {Tylopharynx striata de Man). It may 

 be possible then that by certain developments during the evolution of these 

 trionchiate forms, a partial fusion of the onchi has taken place with an 

 accompanying elongation and narrowing of the pharyngeal cavity giving 

 rise eventually to the hollow pharyngeal stylet. Each of these pharynges 

 has its symmetrical content, but these relations will be considered in a 

 following section in connection with oral structures. 



Oral structures among nematodes 



The oral structures of the free living nematodes are equally as compli- 

 cated as the pharyngeal and show almost as numerous variations. The 

 simple circular mouth, noted in connection with some of the nematodes 

 mentioned in a preceding paragraph as possessing no pharynx is, one may 

 say, the simplest from a structural standpoint, but whether it is the most 

 primitive form cannot yet be said. In view of the fact that the esophagus 



