THE ALIMENTARY CANAL 



55 



tract, as the esophagus, the stomach, the small and the 

 large intestines. The lengthening of the system, it should 

 be noted, does not merely increase its capacity, but multi- 

 plies the surface available for the processes of absorption. 

 A few anatomic expressions may well be denned at this 

 time. Anterior, as we shall use the word, means toward 



Fig. 7. I represents a protozoan cell an ameba which has 

 enclosed a particle available for food (F). The particle occupies 

 the center of a clear space or vacuole (V). Undoubtedly it is sur- 

 rounded by a fluid having digestive powers. II is a diagrammatic 

 section through the familiar sea-anemone. There is a relatively 

 huge digestive cavity (S) with a single opening to the exterior (M). 

 Ill suggests the type of alimentary system found in the earthworm 

 and in higher animals. Two openings exist, the mouth (M), de- 

 finitely devoted to the reception of food, and the anus (A), used 

 exclusively for the discharge of wastes. 



the head; posterior, away from the head. Dorsal means 

 toward the back; ventral, toward the front. Right and 

 left have their ordinary use. (In most figures, the subject 

 being viewed from in front, right and left are reversed.) 

 Reference must often be made to the body cavities. These 



