INSECTA 389 



phase the border of the cells has a striated appearance. The same cell 

 may be capable of both absorption and secretion, but the epithelium 

 as a whole often passes through rapid cycles which necessitate the 

 constant supply of fresh cells. These are found (Fig. 291) in the 

 trough of folds or bottom of pits into which the mid gut epithelium 



Fig. 291. A, Longitudinal section of wall of oesophagus of a termite. 

 B, Longitudinal section of mid gut of termite in secretory phase. C, Trans- 

 verse section of mid gut of Blatta in resting phase. After Imms. bm. base- 

 ment membrane; c, chitinous intima; cm. circular muscle; cr. crypt; ep. 

 cellular layer; e. enteric epithelium; l.m. longitudinal muscles; nc. group 

 of regenerative cells; h. striated hem; pm. peritoneal membrane. 



is thrown. In many insects the surface is increased by the formation of 

 long diverticula, the pyloric caeca ^ the cells of which are not in any 

 way different from the rest of the epithelium. These vary greatly in 

 number. Though the mid gut epithelium has not an internal chitinous 

 lining there is a curious chitinous tube, free in its cavity, the peri 



