FREE-LIVING NEMATODES 467 



e helix or spiral being the fundamental form of the main cuticular 

 ter lateral markings that serve so good a purpose in characterizing 

 species. These external markings are undoubtedly in some species 

 connected with internal series of lateral organs arranged in two rows, 

 one along each lateral field, extending throughout the length of the 

 body. One more or less plausible theory concerning the amphids is 

 that which proposes to regard them as breathing organs. It is only 

 very exceptionally that they are known to have sp'ecial direct con- 

 nection with the central nervous system. Such connection would 

 be expected, if, as some suggest, they are organs of sensation. 

 Their apparent homologues found in some parasitic nematodes 

 seem rudimentary. Possibly they are organs of equilibration. 



In describing the digestive system it is necessary to consider the 

 mouth parts, the salivary or mouth glands, the esophagus, the in- 

 testine, and the rectum. Roughly speaking, the mouth parts may 

 be divided into two main groups : those adapted to biting and those 

 adapted to sucking. The various forms of the pharyngeal cavity 

 in the biting group are shown in the adjacent illustrations, together 



None Conoid Concave- Cyathiform Cyathiform, Cylindroid Compound 



conoid then Cylindroid 



FIG. 769. Forms of the pharynx. (After Cobb.) 



with their corresponding nomenclature (Fig. 769) . The formation of 

 the pharynx in the sucking groups is more uniform. The soft-lipped 

 species are intermediate in form and are adapted to seizing and 

 swallowing various microscopic organisms, both plant and animal. 

 The mouth cavity or pharynx is usually more or less strongly 

 lined with cuticula, and often furnished with cuticular parts serving 

 various purposes according to the food habits of the species. Where 

 the lips are muscular and mobile, not infrequently they are sup- 

 plied with rather complicated gripping organs arranged like the 

 jaws of a lathe chuck. This arrangement of the mouth parts is 

 well illustrated in Enoplus; the reverse motion for ripping tissues 

 open is shown in I r onus (Fig. 781). Mononchus (Fig. 782) shows 

 the development of six muscular lips with opposing pharyngeal 



