ANATOMY AND PHYSIOLOGY. 165 
anatomical and physiological, in the gizzard of birds. 
Perhaps the insect arrangement is the more complete 
of the two. For birds have to swallow gravel to 
assist in the comminution of the refractory particles 
which have resisted the action of their mandibles; 
but insects have all the apparatus within themselves. 
The crop and gizzard must not be confused to- 
gether; the one is not a substitute for, nor yet a 
representative of, the other. They originate in two 
quite distinct parts, and may co-exist in the same 
insect. Wasps, like fleas, flies, and butterflies, have 
no gizzard. ‘Their food is for the most part fluid, and 
needs no farther breaking up than the mandibles can 
effect. And the horny lming, which is so highly 
developed in the cricket, is represented in bees and 
wasps only by a three-lobed valve which closes the 
lower orifice of the crop, oii it like the calyx 
at the end of a pear. 
Resuming, at this point, the description of the suc- 
cessive divisions of the alimentary canal :—the tube 
here suddenly contracts, and dips down between the 
two large air-vesicles. Making a half turn in a left- 
handed spiral, that is to say, in the contrary direction 
to that in which most shells turn, it re-appears on the 
surface as the stomach. The physiological importance 
of this section of the canal is evident, at a glance, 
from the large number of air-tubes with which it is 
supplied. Its walls are thick, and thrown into trans- 
verse folds; and its general appearance forcibly recalls 
that of the large intestine of most Vertebrata. As it 
turns to the left side of the insect, in continuation of 
the spiral which we have begun to trace, it disappears 
between the two ovaries, and, narrowing as it goes, 
