74 ANIMAL LIFE 



than three or four times the length of its body, while a 

 sheep has an alimentary canal twenty-eight times as long 

 as its body. The tiger is carnivorous; the sheep her- 

 bivorous. Associated with the different food habits of the 

 two animals is a striking difference in the alimentary 

 canals. Animals like the horse or cat, which chew their 

 food before swallowing it, have a slender oesophagus ; ani- 

 mals like snakes which swallow their food whole have a 

 wide oesophagus. Birds, that have no teeth and hence 

 can not masticate or grind their food in their mouths, usu- 

 ally have a special grinding stomach, the gizzard, for this 

 purpose. And so we might cite innumerable examples 

 of these inconstant or variable characteristics of the ali- 

 mentary canal. On the other hand, the alimentary canals 

 of all the many-celled animals except the lowest agree in 

 certain important characteristics. Each alimentary canal 

 has two openings, one for the ingress of food and one for 

 the exit of the indigestible portions of the matter taken in, 

 and the canal itself stretches through the body from mouth 

 to anus as a tube, now narrow, now wide, now suddenly 

 expanding into a sac or giving off lateral diverticula, but 

 always simply a lumen or hollow inclosed by a flexible mus- 

 cular wall. The inner lining of the wall is provided with 

 secreting and absorbing structures. Indeed, we can reduce 

 the essential characters of the alimentary canal to even 

 more simple features. The organ of digestion or assimila- 

 tion of all the many-celled animals is merely a surface with 

 which food is brought into contact, and which has the 

 power of digesting this food by means of digestive secre- 

 tions, and of absorbing the food when digested. This sur- 

 face is small or great in extent, depending upon the amount 

 of food necessary to the life of the animal and the difficulty 

 or readiness with which the food can be digested. This 

 surface might just as well be on the outside of the animal's 

 body as on the inside, if it were convenient. In fact, it is 

 on the outside of some animals. Among the Protozoa the 



