Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 Ph- (fig. 45), with sharply marked off pharyngeal structures (Trematoda, Turbellaria), or as a tube communicating with the exterior by an anus (fig. 46). In the last case it becomes divided so as to lead to the distinction of three parts—(1) of the fore-gut (oasophagus) for the reception of the food, (2) of the mid-gut for the digestion of the food, and (3) of the hind-gut for the expulsion of the undigested remains of the food. Sometimes the alimentary
Elementary text-book of zoology, tr. and ed. by Adam Sedgwick, with the assistance of F. G. Heathcote elementarytextbo01clau Year: 1892-1893 Ph- (fig. 45), with sharply marked off pharyngeal structures (Trematoda, Turbellaria), or as a tube communicating with the exterior by an anus (fig. 46). In the last case it becomes divided so as to lead to the distinction of three parts—(1) of the fore-gut (oasophagus) for the reception of the food, (2) of the mid-gut for the digestion of the food, and (3) of the hind-gut for the expulsion of the undigested remains of the food. Sometimes the alimentary canal aborts; and, as in the mouthless Protozoa (Opalina), the mouth opening may be absent (Acanthocephala, Cestoda, Rhizoce- phala). In the higher animals, usually, not only is the number of the divisions greater, but their shape and structure becomes more com- plicated. The organs for the seizure of food also become more complicated, and the appendages placed nearest the mouth of ten become modified to subserve this func- tion. A special chamber, the buccal cavity, becomes ^^^^^ marked off from the fore-gut, in front of or within which hard structures, such as jaws and teeth, for the seizure and mastication of the food are placed (Vertebrata, Gastropoda); and into which secretions (salivary) having a digestive function are poured. The masticatory organs are sometimes placed completely outside the body in front of the mouth, and consist of modi- fied limbs (Arthropoda), which in the parasites are metamorphosed into structures for piercing and sucking; or they may have shifted so as to lie entirely within the pharynx (Rotifera, errant Annelids) or in a muscular dilatation of the posterior end of this organ. At this place there is usually developed a widened chamber, the stomach, which by •A FIG. 45. — Alimentary canal of Distomum hepaticum (after R. Leuckart); D, alimen- tary canal; O, mouth. FIG. 43.—Alimentary canal of a young nematode. O, mouth; Oe, fore-gut (res
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