204 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



canal, is still practically in contact with the external surface, and, there- 

 fore, 'still outside the body. To enable it to pass through the walls of 

 the digestive cavity into the circulatory system is the object of diges- 

 tion, and we shall find that this is accomplished by the production of 

 more or less profound chemical changes in the food-constituents. 



The length, capacity, and complexity of the digestive canal are 

 governed by the complexity of the food. Vegetable feeders, therefore, 

 of all classes of animals, have a more highly developed alimentary canal 

 than animals of the same class which feed on animal food. With this 

 modification, the statement may be made that there is a gradual in- 

 crease in complexity of the digestive organs as the animal scale is 

 ascended. 



In the amoeba we find the simplest possible representative of a 

 digestive function. When alimentary substances come in contact with 

 1 2 3 



Fig. 56.— Paramecium bursaria, after Stein. (Huxley.) 



1 . The animal viewed from the dorsal side. A, cortical layer of the body ; B, nucleus ; C, contractile 

 chamber ; D D', matters taken in as food ; E. chlorophyll granules. 



2. The animal viewed from the ventral side. A. depression leading to B, mouth ; C, gullet ; D, 

 nucleus; D', nucleolus ; E. central sarcode. In both these figures the arrows indicate the direction of 

 the circulation of the sarcode. 



3. Paramajcium dividing transversely. A A', contractile spaces; B B', nucleus dividing; 

 C C, nucleolus. 



the soft external surface of the amoeba a temporal - }' depression or 

 pocket forms around them, and by the gradual deepening of this de- 

 pression and closing of a wall around it a cavity is formed and the 

 alimentary substances gradually brought to the interior of the mass. 

 Within this temporary chamber the alimentary substances are removed 

 and appropriated, while the undigestible residue is removed by a process 

 the reverse of that concerned in its introduction. The amoeba there- 

 fore has, strictly speaking, no digestive organs, but it temporarily de- 

 velops a cavity at the point of contact with the food. 



In certain of the infusoria, such as the paramceeium, we have a 

 single portion of the external body surface specialized as the orifice of 

 entrance for the food-stuffs. An oral aperture (which in this illustration 



