THE LIVEK AND THE TEETH. 87 



The stomacli and intestine are invested by a peritoneal 

 membrane, and connected, by mesogastric and mesenteric folds 

 of that membrane, with the median dorsal wall of the abdo- 

 minal cavity. Glands appertaining to the lymphatic system 

 frequently abound in the mesenteric folds, and a highly 

 vascular gland of this system, the spleen, is always (except 

 in Amphioxus, Myxine, and the Lepttoceplialidoi) developed 

 in close proximity to the stomach. A pancreatic gland very 

 generally pours its secretion into the anterior end of the 

 intestine. Salivary glands very commonly open into the 

 mouth ; and, in the higher Vertebrata, anal glands are not 

 unusually developed in connection with the termination of 

 the rectum. 



The structures connected with the alimentary canal of 

 vertebrate animals, which are most characteristic and pecu- 

 liar, are the liver and the teeth. 



The Liver. — In invertebrate animals this organ is always 

 ultimately resolvable into cascal tubes, the ends of the 

 hepatic ducts, which are lined with an epithelium, and not 

 reticulated ; and it has no receptacle for the bile. In most 

 Vertebrata the ends of the hepatic ducts have not been 

 satisfactorily traced, nor is it certain that the immense pro- 

 portional mass of hepatic corpuscles is contained in tubes 

 continuous with them : if such be the case, the tubes must be 

 reticulated. The ducts of the vertebrate liver very frequently 

 pour the bile, directly or indirectly, into a receptacle, the gall- 

 bladder. Amphioxus stands alone among vertebrated animals, 

 in having a cscal diverticulum of the intestine for a liver. 



The Teeth. — Teeth, in Mollusca and Annulosa, are always 

 " ecderonic," cuticular, or epithelial structures. In Verte- 

 brata true teeth are invariably " enderonic," or developed, 

 not from the epithelium of the mucous membrane of the 

 alimentary canal, but from a layer between this and the 

 vascular deep substance of the enderon, which answers to 

 the dermis in the integument. The horny "teeth" of the 

 Lampreys, and of Omithorhynchus, appear to be ecderonic 



