164 NATURAL HISTORY OF WASPS. 
The mode of arrangement of the bands which form 
the muscular coat of the intestines varies according 
to the requirements of the part. In the cesophagus 
they affect a longitudinal direction, in the next 
division of the tube the direction is mainly circular, 
becoming again longitudinal in the lower bowel. 
Outside the muscular is the peritoneal coat, a thin 
transparent membrane, rarely visible under ordinary 
circumstances, but occasionally to be traced, when 
the stomach or intestines are thrown into wrinkles, 
passing across from fold to fold. A mucous coat 
Imes the whole throughout. This is generally thick 
and soft, and the changes which the food undergoes 
in the alimentary canal are mainly effected by its 
agency. In some insects the mucous membrane is 
covered, at certain parts, with a fourth layer of a 
horny nature.. This is not found in wasps; but is 
_ most perfectly developed in the teeth with which 
the gizzard of the Orthoptera, instanced in crickets 
and cockroaches, is armed. These successive layers 
are the exact counterpart of what we find m the 
alimentary canal of the Vertebrata. There is a 
serous, a muscular, and a mucous layer, each extend- 
ing down the entire length of the capillary imtes- 
tinal canal, more or less developed severally in diffe- 
rent parts according to the function of those parts. 
The identity of the serous layer with the peritoneum 
of the higher animals is questionable ; as the peritoneal 
cavity of insects is rather a collection of vascular 
sinuses than a simple serous sac. But the muscular 
and mucous coats are strictly analogous to those of 
the higher animals. And the horny epithelium of the 
gizzard of Orthoptera finds its counterpart, both 
