ANATOMY AND PHYSIOLOGY I2.t 
and chlorophyll. The rectum terminates in the anus, which 
opens through the last segment of the abdomen, always above 
the genital aperture. 
Histology.—The epithelial wall of the alimentary tract is 
a single layer of cells (Fig. 151), which secretes the intima, 
or lining layer, and the basement membrane 
tureless enveloping layer. 
a delicate, struc- 
The intima, which is contin- 
uous with the external cutic- 
ula, 1s chitinous in the fore 
and hind gut (which are 
ectodermal in origin), but 
not in the mid gut (entoder- 
mal), and usually exhibits 
extremely fine transverse 
striz, which are due prob- 
ably to minute pore canals. 
Surrounding the basement 
membrane is a series of cir- Wall of mid intestine of silk worm, 
cular muscles and outside transverse section. b, basement membrane; 
p 5; y is ie’ i . c, circular muscle; 7, intima; J, longitudinal 
these 18°23 laver of longitudi- muscle; , mn, nuclei of epithelial cells; s, 
° a iS secretory cell. 
nal muscles. The circular 
muscles serve to constrict the pharynx in sucking insects 
and, in general, to squeeze backward the contents of the 
alimentary canal by successively reducing its caliber. The 
longitudinal muscles, restricted almost entirely to the mid 
intestine, act in opposition to the constricting muscles to en- 
large the lumen of the food canal and in addition to effect 
peristaltic movements of the stomach. 
The intima of the crop is sometimes shaped into teeth, and 
that of the proventriculus is heavily chitinized and variously 
modified to form spines, teeth or ridges. 
Salivary Glands.—In their simplest condition, the salivary 
glands are a pair of blind tubes (Fig. 152), one on each side 
of the cesophagus and opening separately at the base of the 
hypopharynx. Commonly, however, the glands open through 
