132 m. straus-durckheim's 



crop, or ventricule; and where these do not exist, to the 

 gizzard. 



The part which follows the gizzard constitutes the intestine, 

 and is divided into three parts, the duodenum, colon, and 

 rectum. The two latter often differ only in size ; sometimes, 

 however, the rectum is dilated so as to form a caecum, as in 

 Aranea, Nepa, Dyticus, &c. But in the Myriapoda and 

 Crustacea we find no difference in these two parts. The 

 duodenum is not always distinct, though its limits are mostly 

 marked by the insertion of the biliary vessels. 



Intimate Structure of the Alimentary Canal. 



The alimentary canal consists of three tunics, or coats ; the 

 most internal is a mucous membrane, analogous to the villous 

 coat of Vertebrata; it is merely a prolongation of the integu- 

 ments ; is very distinct in the oesophagus, ventriculus bul- 

 bosus, and rectum; less so in the ventriculus ; and is very 

 distinct in those species where the integuments are solid. 

 The second tunic {membrane propre) is every where easily 

 distinguishable : generally it is white, and very thin ; some- 

 times, however, it is thick, and of a spongy texture. It 

 presents excessively small granulations, which have been 

 considered as the mouths of the absorbents ; but it is more 

 probable that they are gastric glands. The third is the 

 muscular coat, which only clothes certain parts, as the intes- 

 tines and gizzard, sometimes the oesophagus, and yet more 

 rarely the ingluvies and ventriculus. 



We find in the articulated animals no true peritoneum; but 

 the viscera of the Arachnida are retained in place by trans- 

 verse fibrous septa, of a loose texture, which pass perpendi- 

 cularly from the intervals of the segments, being, as it were, 

 so many diaphragms. The viscera of Crustacea are con- 

 nected by a loose cellular tissue ; those of Insects and Myria- 

 poda are retained in place by the trachece. 



Secretory Glands dependent upon the Alimentary Canal. 



The difference of form and structure which we find in the 

 glands of different species, is due principally to the difference 

 in the mode of circulation of the blood in different classes. 



