PARASITIC ROUNDWORMS 



513 



be reduced to a few fundamental types (Fig. 812). In the first 

 the tip of the body is unarmed or at most provided wiili a few 

 minute papillae arranged around the mouth opening wliich is a 

 minute circular orifice. In a second, three lips are present, a large- 

 dorsal and two smaller ventro-lateral, which border a l.rian;;ular 

 mouth. In a third, the oral aperture is a dorso-ventral slit guar(le<l 

 by two lateral jaws often called lips but very distinct in form and 

 function from the triple labia of the second type. In the fourth 

 class one finds a hollow cup-shaped capsule with an entire margin 

 which in lateral aspect resembles the jaws of the third type but is 

 very unlike them in general plan. The capsule is a powerful 

 sucking organ, the jaws act as a grasping organ like a vise or pin- 

 cers, the lips are weaker and more varied in movement. These 

 main types of oral apparatus are modified in so manv directions 

 that it is often difficult to comprehend the general t}'pe inNoKcd 

 in a compKcated case. 



The mouth cavity may be tubular, funnel-shaped, or even ex- 

 panded into a globular or oval capsule or pharynx. Following 

 this region comes the esophagus which is either muscular or 

 capillary. The muscular type is prominent, thick walled, and tri- 

 angular in cross section (Fig. 813, a), with the muscle fibers perjKMi- 

 dicular to the lumen. By the contraction of these tibrrs the 

 cavity is enlarged and the a 



organ acts as a pump to 

 draw in food. The esophagus 

 may be differentiated into 

 two regions, one clearly mus- 

 cular and the other granu- 

 lar, or the single muscular 

 region may have large (sali- 

 vary?) gland cells in its wall. 

 It is frequently terminated by a spherical bulb which contain>> a 

 valvular apparatus. In some cases this bulb is double. The 

 cavity is fined by an inturned layer of the external cuticula wliieh 

 terminates at the bulb. This is the type of esophagus found in 

 free-living forms (see Chapter XV, p. 461, Fig. 766). The 

 capifiary esophagus (Fig. 813, b), consists of a minute clfitinous tube 



Fig. 813. a, Aiuylosiomu diunlcnaU. Trans-srction vi 

 the esophagus, magnified, (.\ftcr Looss.) b. Truknuima 

 conlortum. Transverse section of esophagus, magnified. 

 (After von Linstow.) 



