188 DIGESTIVE APPARATUS—GASTRIC DIGESTION. 
tary tube ; and it is not easy to say where it commences and 
terminates. In Spiders and Scorpions, too, which live upon 
the juices they suck from other animals, the alimentary tube 
is very simple ; and it is scarcely dilated into a proper sto- 
mach. And in most of the Radiated classes, we find the 
stomach to possess only one orifice, through which the undi- 
gested residue of the food is cast out, as well as fresh sup- 
plies taken in. But this stomach is not always a simple 
bag ; thus in the Star-jish it sends prolongations into the 
rays, the use of which is at present un- 
determined. There are certain animals in 
which no digestive cavity exists: their 
sustenance being derived either from the 
juices prepared by other animals, in 
whose tissues or cavities they are im- 
bedded, and being introduced by absorp- 
tion through the whole surface, as is the 
case in the lower Hntozoa (fig. 53); or 
from particles which are drawn into the 
midst of the soft gelatinous substance of 
their bodies, and undergo a sort of diges 
tion there, as is the case with the Rhzzo- 
poda (§ 129). 
Gastric Digestion :—Chymification. 
204. The food which has been re- 
duced in the mouth by the action of the 
teeth, or in the stomach itself by the 
movement of its own tendinous walls, is 
prepared for the real process of digestion; 
by which it is converted into a fluid, 
and thus made fit to be truly received 
into the system, by being absorbed into 
its vessels. The chief agent in the 
singin digestive process is a fluid termed the 
GastnicFouncurs, gastric juice, which is secreted or sepa- 
as seen in a vertical section rated from the blood by a vast number 
of the Stomach, megniteg Of little bags or follicles (fig. 113),im- 
twenty diameters ats. bedded in the walls of the stomach. 
When the cavity is empty, this fluid is secreted in very small 
quantities ; but, like the salivary secretion, it is poured out 
