127] COMPARATIVE STUDIES ON NEMATODES— BETHERINGTON 23 



degree. Also the genera lonema, Schistodera, and Nemanema of Cobb, 

 and Thoracostoma Marion have this simple mouth and lack a pharynx. 



Next in simplicity are nematodes which not only have a simple circular 

 mouth with none or amalgamated Lips, {Terschellingia longicaudata, 

 Monhystera stenosoma) but also possess a pharynx. The pharynx is one 

 structure which is subject to the greatest diversification and is to a great 

 extent indicative of height of specialization and adaptation in free living 

 forms and also among parasitic species. 



N emanema simplex Cobb, a marine algae-inhabiting nematode, exhibits 

 a very simple cephalic region; bristles are absent; the mouth is a round ori- 

 fice, surrounded by possibly six exceptionally indefinite papillae, and the 

 pharynx is the merest conoid suggestion. This pharynx, almost unnotice- 

 able in the form above, may become greatly elongated {Rhynchonema 

 cindum Cobb) and constant in width; short and narrow {Litoncma nudum 

 Cobb); or cavernous and greatly modified as in the genus Mononchus 

 Bastian (Fig. 12) notably, and also in many others. The very undififeren- 

 tiated conoid pharynx and its derivatives attained chiefly by elongation are 

 prominent in nematodes living on a more or less liquid diet. In general, 

 too, the enlarged and widened pharynges are greatly modified by armatures 

 of onchi derived from their walls. Such are to be found among nematodes 

 living upon a solid diet demanding more or less maceration. A very special 

 type of such a pharynx is found among the spear-bearing nematodes 

 obtaining food by piercing and then sucking the fluids, for example, from 

 the roots and tender shoots of either water or land inhabiting plants. 



There are, of course, intergradations among the pharynges such that 

 one may arrange a series beginning with nematodes possessing no pharynx 

 (Litotes) followed by forms with larger but unarmed pharynx which in 

 turn pass into other forms bearing teeth among which the simpler ones 

 bear three teeth, one each in a position corresponding to the respective 

 sectors of the esophagus {Mesonchium poriferum Cobb). The tooth- 

 bearing type, perhaps by further modification, passes into the spear-bearing 

 nematodes such as Dorylaimus, Heterodera, Dorylium, Eutylenchus, 

 Tylenchorhynchus, etc. Among the Mononchs the dorsal tooth is very 

 large and works in opposition to the teeth belonging to the subventral 

 sectors of the pharynx. The teeth on these last two portions may be very 

 small and there may be only one per section as in Mononchus radialus; 

 they may be small and accompanied by numerous denticles (M. dentatus 

 Cobb), or by two rasp-hke structures approximated near the mid ventral 

 line of the pharynx so that they may operate against the large dorsal tooth 

 {M. muscorum Bastian); or further there may be no ventral onchi at all 

 {M. zschokkei Mengel). The pharynx is partially mobile in some forms by 

 means of three seams or hinges, one ventral and two lateral and a bit dorsal. 

 In other cases where the walls of the cavity appear immobile, the lips seem 



