Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 56 OEGA>' A>'D DEVELOPMENT OF AJSIMALS IN GENEUAX. (fig. 45), with sharply marked off pharjmgeal structures (Trematoda, Turbellaria), or as a tube communicating with the exterior by an anus (tig. 46). In the last case it becomes divided so as to lead to the distinction of three parts—(1) of the fore-gut (oesophagus) for the reception of the food, (2) of the mid-gut for the digestion of the food, and (3) of the hind-gut for
Elementary text-book of zoology, general Elementary text-book of zoology, general part and special part: protozoa to insecta elementarytextbo00clau Year: 1892 56 OEGA>' A>'D DEVELOPMENT OF AJSIMALS IN GENEUAX. (fig. 45), with sharply marked off pharjmgeal structures (Trematoda, Turbellaria), or as a tube communicating with the exterior by an anus (tig. 46). In the last case it becomes divided so as to lead to the distinction of three parts—(1) of the fore-gut (oesophagus) 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 -5 (Acanthocephala, Cestoda, E,hizoce- phala). In the higher animals, usually, not only is the number of the divisions greater, but their shape and struct ui'e becomes more com- plicated. The organs for the seizure of food also become more complicated, and the appendages placed nearest the movith often become modified to subserve this f unc- 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 (Vei^tebrata, 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 (Ai'thropoda), 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 Fig. 45. — Alimentary canal of Distoinum hepaticum (after R. Leuckart); D, alimen- tary caual; O, mouth
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