THE (ESOPHAGUS, STOMACHS AND INTESTINES. 419 



sheath, which he entirely overlooked, although I drew atten- 

 tion to them seven years before his monograph appeared [163]. 

 Indeed, he does not seem to have read my paper on the 

 subject, at least he makes no reference to it. 



There are a number of minute hollow projections on the 

 external sheath, the extremities of which are only closed by the 

 large secreting cells. Each of these projections is overhung and 

 protected by a small group of setae, usually five or six in 

 number ; they are evidently the excretory orifices of the 

 papilla. 



The margin of the external sheath is strengthened by a 

 crenated edge, into which the muscular coat of the rectum is 

 inserted on one side, and the radiating muscles of the base of 

 the papilla on the other. 



The Internal Sheath consists of adenoid tissue formed by 

 branching cells ; it is continuous with the peritoneal coat of 

 the rectum ; and is connected by fibres which penetrate the 

 muscular tissue with the basement membrane beneath the 

 muscles. It is also apparently inserted into the chitinous 

 margin of the external sheath, thus separating the epithelium 

 of the rectal pouch from the columnar cells of the papilla. 



Chun describes the cuticle of the rectum as splitting into 

 two layers, one forming the external and the other the internal 

 sheath of the papilla, but he believed the rectum to be devoid 

 of epithelium. In the young imago and in the nymph it is 

 easy to see the transition of the flat pavement cells into the 

 great cylindrical cells of the papilla, which are clearly a modifi- 

 cation of the rectal epithelium. I cannot, therefore, regard the 

 internal sheath as a portion of the cuticular lining of the 

 rectum. In the young state it is indubitably continuous with 

 its peritoneal coat ; neither is the internal sheath a cuticular 

 membrane. 



The Tracheal Vessels. Chun says that each papilla receives 

 two tracheal trunks. There is no constancy in the number of 

 cylindrical tracheae which enter each papilla, but they invariably 

 arise from a single large tracheal trunk. There is not, there- 

 fore, an efferent and an afferent trachea, a point of great 



