56 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



(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 (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 



(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 



FIG. 45. Alimentary 

 canal of Distomum 

 hepaticum (after R. 

 Leuckart) ; D, alimen- 

 tary canal ; 0, mouth. 



FIG. 43. Alimentary canal of 

 a young nematode. O, mouth ; 

 Oe, fore-gut (oesophagus) with 

 pharyngeal dilatation, Ph ; D, 

 mid-gut; A, anus. 



