298 ANIMAL PHYSIOLOGY. 
which is carried so far among the higher vertebrates. But, as 
recent investigations have shown, such intra-cellular digestion 
exists to some extent in the alimentary canal of the highest 
members of the vertebrate group (see page 345). 
The means for grasping and triturating food among in- 
vertebrates are very complicated and varied, as are also those 
adapted for sucking the juices of prey. Examples to hand are 
to be found in the crab, crayfish, spider, grasshopper, beetle, 
Fic. 256.—Diagram illustrating arrar t of intestine, nervous system, etc., in common 
snail, Helix (after Huxley). m, mouth; t, tooth; od, odontophore ; g, gullet; ¢, crop; 
s, stomach ; 7, rectum ; a, anus; r. 8, renal sac; h, heart; 1, lung (modified pallial cham- 
ber); ”, its external aperaure ; em, thick edge of mantle united with sides of body; /, 
foot ; cpg, cerebral, pedal, and parieto-splanchnic ganglia aggregated round gullet. 
etc., on the one hand, and the butterfly, house-fly, leech, etc., 
on the other. , 
The digestive system of the earth-worm has been studied 
with some care. It illustrates a sort of extra-corporeal diges- 
tion, in that it secretes a fluid from the mouth which seems to 
act both chemically and mechanically on the starch-grains 
of the leaves on which it feeds. It is provided with an organ 
in which, as with birds, small stones are found, so that. the 
imperfections of its mouth are compensated for by this gizzard ~ 
which triturates the food. Its calciferous glands supply the 
alkaline fluids necessary to neutralize the humus acids of de- 
caying leaves, for intestinal digestion only proceeds in an alka- 
line medium. 
The gastric mill of a crab (Fig. 228) is a provision of ob- 
vious value in so voracious a creature. 
