DIGESTION OF FOOD. 295 
another, where a different fluid is secreted, would be expected. 
As a matter of fact, mechanical and chemical forces play a 
large part in the actual preparation of the food for absorption. 
Behind these lie, of course, the vital properties of the glands, 
which prepare the active fluids from the blood, so that a study 
of digestion naturally divides itself into the consideration of— 
x ; { 
Fic. 252,—A thea pycnogonides, a marine animal (after Quatrefages). oc, cesophagus: 
“a, antennee ; s, stomach, with prolongations into antenns rei limbs @. er 
1. The digestive juices; 2. The secretory processes; and, 3. The 
muscular and nervous mechanism by which the food is carried 
from one part of the digestive tract to another, and the waste 
matter finally expelled. 
Fia. 253.—Longitudinal vertical section of body of leech, Hirudo medicinalis (after Leuckart). 
a, mouth ; b, b, b, sacculations of alimentary canal; c, anus; d, terminal sucker; e, cere- 
bral ganglia; /, f’, chain of post-cesophageal ganglia ; g, g, g, segmental organs. 
Embryological—_ The alimentary tract, as we have seen, is 
formed by an infolding of the splanchnopleure, and, according 
as the growth is more or less marked, does the canal become 
