468 FRESH-WATER BIOLOGY 



teeth used in seizing prey. There are a number of genera in which the 

 pharynx is armed with from one to three prominent teeth of prob- 

 lematical function. In some of these cases the teeth are the outlets 

 of an equal number of glands located in the wall of the esophagus. 

 The secretions of these glands are probably saUvary in nature, or 

 possibly in some cases venomous, or even, as has been suggested, 

 excretory. These suppositions rest on structural and food-habit 

 considerations, rather than on an examination of the nature of 

 the secretions. The saliva theory is strongly supported by the 

 nature of these glands, whether their form, number, position, or 

 structure is considered, but they sometimes empty through fang- 

 like projections in carnivorous species that one would think could 

 profit by the use of venom in much the same way that serpents do. 



The nematode esophagus is an organ of which every cross-section 

 is usually substantially circular, though the diameter may vary 

 much in the various parts. The central canal is usually trique- 

 trous in cross-section (Fig. 766). The lining is uniformly cuticular 

 and varies considerably in thickness in the various species. In the 

 simple cylindrical form of esophagus, radial muscles, the contraction 

 of which accomphshes the act of swallowing, everywhere pass from 

 the lining of the organ to the exterior cyHndroid wall. The action of 

 these muscles is peristaltic, first creating the necessary suction, and, 

 after the food is sucked in, rapidly forcing it along toward the intes- 

 tine. The act of swallowing is often lightning-like in its rapidity. 



In addition to this general radial musculature the esophagus some- 

 times presents spherical or ellipsoidal muscular swellings, or bulbs, 

 often suppHed with a central cuticular valve, for exerting more pow- 

 erful suction than could be produced by the narrower tubular part. 

 The presence of bulbs denotes certain methods of feeding, — either 

 the lips need to be fastened securely to the source of food in order 

 to facilitate the stabbing action of the oral spear, or it is necessary 

 to exert unusual suction in order to ingest the food. There may be 

 one, two, or three of these bulbs, or none. The corresponding forms 

 of the esophagus are shown in the accompanying illustration 

 (Fig. 770), to which the appropriate names are appended. In rare 

 cases the esophagus is not clearly marked off from the intestine, 

 but there nearly always exists between these two parts of the ali- 



