120 



ANATOMY. 



96. 



The entire intestinal canal consists of three skins, or layers of mem- 

 brane. 



The innermost membrane (PL XVII. f. 1), which may be considered 

 as a continuation of the exterior epidermis, is very smooth and texture- 

 less, and only sometimes longitudinally folded, and armed above with 

 horny lines, ridges, or teeth (PI. XVII. f. 2. 5 7)- It is particularly 

 distinct in the pharynx, crop, and proventriculus, the horny teeth of the 

 latter being formed by it. This internal membrane is most apparent 

 in insects with hard cases, as the Coleoptera and Orthoptera, whereas 

 it is not so evident in the haustellate Diptera and Lepidoplera. From 

 the proventriculus it forms a very delicate perfectly uniform covering, 

 and generally occupies less compass than the other intestinal mem- 

 branes. We here call it the epidermis, it being its analogue, or pro- 

 perly, the mucous membrane, as it corresponds with the tunica mucosa 

 of the superior animals. 



The second layer, which we call with Straus the PROPER skin (inem- 

 brana propria), is white and smooth, and usually thin, but sometimes 

 thicker and spongy, most frequently without any texture, but occa- 

 sionally figured (Hydrophilus, PI. XVII. f. 2.). This membrane, 

 which Ramdohr treats as a layer formed of transuded chyle, is pecu- 

 liar to the intestinal canal, and is not found in the other internal organs ; 

 it may therefore be considered as a continuation of the second layer of 

 the exterior integument, of which we shall treat below. Indeed, the 

 space between the mucous membrane and this peculiar skin, which is 

 very considerable in the stomach, and particularly in caterpillars, is 

 often occupied by a flocky web, formed of transuded chyme, and this 

 may have misled Ramdohr in his idea of it. According to Straus, horny 

 prominences are sometimes observed in this intermediate skin, parti- 

 cularly in the vicinity of the stomach, which might be considered as 

 absorbing pores, but which Straus, perhaps more correctly, treats as 

 glands, and they are therefore called gastral glands (glandules gastric fe^). 

 I have observed these organs only upon the inner surface of the mus- 

 cular membrane, but particularly distinct in Hydrophilus, in which 

 insect the long cylindrical stomach is completely and regularly covered 

 with such glands, which consist of a transparent case inclosing a darker 

 kernel (PI. XVII. f. 3.). 



The third layer (PI. XVII. f. 3 and 4.) is a compact, firm, fleshy 



