THE G@SOPHAGUS, STOMACHS AND INTESTINES. 419 
sheath, which he entirely overlooked, although I drew atten- 
tion to them seven years before his monograph appeared [168]. 
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 sete, 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 tracheze 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 
