THE ALIMENTARY CANAL AND ITS APPENDAGES. 115 



Considered with respect to its mode of formation, the alimen- 

 tary canal of all but the very simplest animals falls into three 

 sections viz., (1) the mesenteron, or primitive digestive cavity, 

 lined by hypoblast ; (2) the stomodseura, or mouth-section, 

 lined by epiblast, continuous with that of the external surface ; 

 and (3) the proctodacum, or anal section, lined by epiblast 

 folded inwards from the anus, just as the epiblast of the 

 stomodaoum is folded in from the mouth. The mesenteron of the 

 Cockroach is very short, as in other Arthropoda, and includes 

 only the chylific stomach with its diverticula. The mouth, 

 oesophagus, and crop form the stomodaeum, while the proc- 

 todaeum begins with the Malpighian tubules, and extends 

 thence to the anus. Both stomodaDum and proctodaeuin have 

 a chitinous lining, which is wanting in the mesenteron. At the 

 time of moult, or a little after, this lining is broken up and 

 passed out of the body. 



The mouth of the Cockroach is enclosed between the labrum 

 in front, and the labium behind, while it is bounded laterally 

 by the mandibles and first pair of maxillae. The chitinous 



Mo Ml 



Fig. 57. Section of "\Vall of Crop. Cc, chitinous layer ; C, chitinogenous cells ; 

 Mi, inner muscular layer ; Mo, outer do. X 275. 



lining is thrown into many folds, some of which can be 

 obliterated by distension, while others are permanent and filled 

 with solid tissues. The lingua is such a permanent fold, lying 

 like a tongue upon the posterior wall of the cavity and reaching 

 as far as the external opening. The thin chitinous surface of 

 the lingua is hairy, like other parts of the mouth, and stiffened 

 bv special chitinous rods or bands. The salivary ducts open by 

 a common orifice on its hinder surface. Above, the mouth leads 

 into a narrow gullet or oesophagus, with longitudinally folded 

 walls, which traverses the nervous ring, and then passes 

 through the occipital foramen to the neck and thorax. Here it 



