ANATOMY AND PHYSIOLOGY 



121 



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 a delicate, struc- 

 tureless enveloping layer. 

 The intima, which is contin- 

 uous with the external cutic- 

 ula, is 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 

 striae, which are due prob- 

 ably to minute pore canals. 

 Surrounding the basement 

 membrane is a series of cir- 

 cular muscles and outside 

 these is a layer of longitudi- 

 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 oesophagus and opening separately at the base of the 

 hypopharynx. Commonly, however, the glands open through 



-I 



Wall of mid intestine of silk worm, 

 transverse section, b, basement membrane; 

 c, circular muscle; i, intima; /, longitudinal 

 muscle; n, n, nuclei of epithelial cells; s, 

 secretory cell. 



