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 esophagus; ani- 
mals like snakes which swallow their food whole have a 
wide csophagus. 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 
