CHAPTER VI 

 THE ALIMENTARY CANAL 



THE single-celled animal digests its food within its own 

 protoplasm, sometimes holding it for a while in a temporary 

 cavity filled with fluid, the so-called food vacuole. In 

 such intracellular cavities true digestive secretions contain- 

 ing enzymes are doubtless at work. It is probable that 

 single-celled forms may also secrete enzymes to the exte- 

 rior and so modify food material which is near-by, but not 

 yet enclosed. This appears to be the case with bacte- 

 ria when they dissolve the solid gelatin in which they are 

 growing. 



Among many-celled animals digestion of this second 

 type, that is, external to the cells, becomes more con- 

 spicuous. Their bodies are so formed as to contain spaces 

 in which food may undergo digestion and from which the 

 hydrolyzed products may be absorbed. In the sea- 

 anemone a round opening or mouth leads to a cavity which 

 is very large in proportion to the size of the animal. 

 This primitive alimentary tract has no other opening. 

 In the earthworm, a somewhat more highly developed 

 form, a straight canal in the axis of the body leads from 

 a mouth near the anterior end to an anus at the posterior. 

 However much the alimentary systems of the higher ani- 

 mals may be elaborated, each still represents a more or less 

 winding passage between a mouth through which food is 

 received and a vent or anus for the discharge of residues and 

 excretions. The canal may be greatly lengthened through 

 coiling. Some sections may be widened and others nar- 

 rowed; the walls in some places may be thick and elsewhere 

 thin. Local differentiation of this kind causes us to dis- 

 tinguish in the human subject the familiar divisions of the 



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