. Fresh-water biology. Freshwater biology. 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 with a few minute papillae arranged around the mouth opening which is a minute circular orifice. In a second, three lips are present, a large dorsal and two smaller ventro-lateral, which border a triangular mouth. In a third, the oral aperture is a dorso-ventral slit guarded by two lateral jaws often called Ups but very distinct in form and function from the triple labia of the second type. In the fourth class one
. Fresh-water biology. Freshwater biology. 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 with a few minute papillae arranged around the mouth opening which is a minute circular orifice. In a second, three lips are present, a large dorsal and two smaller ventro-lateral, which border a triangular mouth. In a third, the oral aperture is a dorso-ventral slit guarded by two lateral jaws often called Ups 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 Ups are weaker and more varied in movement. These main types of oral apparatus are modified in so many directions that it is often difficult to comprehend the general type involved in a compHcated 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. 813,0;), with the muscle fibers perpen- dicular to the lumen. By the contraction of these fibers the cavity is enlarged and the 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 contains a valvular apparatus. In some cases this bulb is double. The cavity is lined by an inturned layer of the external cuticula which terminates at the bulb. This is the tj^e of esophagus found in free-Hving forms (see Chapter XV, p. 461, Fig. 7
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectfreshwa, bookyear1918
