i26 



ALIMENTARY SYSTEM 



rectum. The total length of the alimentary canal is extremely 

 variable ; it is necessarily at least as long as the distance between 

 the mouth and anal orifice, but sometimes it is five or six 

 times as long as this, and some of its parts then form coils in 

 the abdominal cavity. 



The alimentary canal has two coats of muscles : a longitudinal 

 and a transverse or annular. Both coexist in most of its parts. 

 Internal to these coats there exists in the anterior and posterior 

 parts of the canal a chitinous layer, which in the stomach is 

 replaced by a remarkable epithelium, the cells of which are 

 renewed, new ones growing while the old are still in activity. 

 We figure a portion of this structure after Miall and Denny, 

 and may remark that Oudemans ^ has verified the correctness of 

 their representation. The layers below represent the longi- 

 tudinal and transverse muscles. 



Fig. 70. Epithelium of stomach 

 of Cockroach (after Miall and 

 Denny) : the lower parts indi- 

 cate the transverse and longi- 

 tudinal muscular layers. 



In addition to the various diverticula we have mentioned, 

 there are two important sets of organs connected with the 

 alimentary canal, viz. the salivary glands and the Malpighian 

 tubes. 



The salivary glands are present in many Insects, but are 

 absent in others. They are situate in the anterior portion of 

 the body, and are very variable in their development, being 

 sometimes very extensive, in other cases inconspicuous. They 

 consist either of simple tubes lined with cells, or of branched 

 tubes, or of tubes dilated laterally into little acini or groups of 

 bags, the arrangement then somewhat resembling that of a bunch 

 of grapes. There are sometimes large sacs or reservoirs con- 

 nected with the efferent tubes proceeding from the secreting 

 portions of the glands. The salivary glands ultimately discharge 

 into the mouth, so that the fluid secreted by them has to be 

 ^ Bijd. DierlcMnde, 16, 1888, p. 192. 



