ALIMENTARY CANAL. 



531 



the simple narrower portion which follows it. Larger caeca, too, 

 after the manner of hepatic glands, may be inserted at the com- 

 mencement of the chylific ventricle (Orthoptera). 



The commencement of the hind gut or posterior portion of the 

 alimentary canal is indicated by the opening of filiform csecal tubes, 

 the Malpighian vessels. It is divided into two or more rarely three 

 .regions, which are distinguished as the small intestine, the large 

 intestine and the rectum. The last region is provided with a strong 

 layer of muscles, and contains in its walls four, six or more longitudinal 

 ridges, the so-called rectal glands (fig. 439, R). Sometimes two 

 glands, the so-called anal glands (G.Dr, 

 Ad), open into the rectum immediately in 

 front of the anus. Their secretion, on 

 account of its irritating qualities and dis- 

 agreeable smell, seems to serve as a pro- 

 tection to the animal. In exceptional 

 cases the larva alone takes up nutriment, 

 the sexually mature apterous form being 

 without a mouth (Ephemera). Finally 

 the stomach of the larva in a few cases 

 ends blindly, and does, not communicate 

 with the hind gut (larvae of Hymenoptera, 

 Pupipara, Ant-lion). 



The Malpighian vessels already men- 

 tioned, which \vere formerly erroneously 

 held to be bile organs, undoubtedly func- 

 tion as urinary organs. Their contents, 

 secreted by the large nucleated cells of 

 their walls, are usually of a brownish 

 yellow or white colour, and consist of an 

 aggregation of small granules and con- 

 cretions, which, for the most part, consist 

 of uric acid. Crystals of oxalate of lime 

 and taurin have also been found. The 

 numbers and grouping of these filiform 

 tubes, which are usually very long and wound round about the chy- 

 lific ventricle, varies very much. As a rule there are four or six, 

 or more rarely eight of them opening into the intestine, but in the 

 Hymenoptera and Orthoptera the number is much larger; in the 

 latter there may even be a common duct into which the tubes are 

 united (Gryllotalpa). 



FIG. 440. Alimentary canal and 

 glandular appendages of a 

 Beetle (Carabus) (after L. Da- 

 four). Oe, oesophagus; Jn, 

 crop ; Pv, proventriculus ; 

 CM, chylific ventricle; My, 

 Malpighian tubes ; R, rectum ; 

 Ad, anal glands with vesicle. 



