THE LIVER AND THE TEETH. 79 



gans of the lateral line" or " mucous canals ; " and they 

 were formerly supposed to be the secretory glands of the 

 slimy matter which coats the bodies of fishes, and which is 

 really modified epidermis. 



The Alimentary Canal. — This part of vertebrate organi- 

 zation always exhibits a differentiation into mouth, pharynx, 

 oesophagus, stomach, and intestine ; and the last has always 

 a median, or nearly median, aperture on the ventral surface 

 of the body. It may open by itself ; or into a cloaca, or cham- 

 ber common to it, the urinary and the genital organs. 



The intestine is generally distinguishable into small and 

 large ; and, at the junction of the two, one or two cceca are 

 frequently developed from the former. 



The stomach 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 abdomi- 

 nal cavity. Glands appertaining to the lymphatic system 

 frequently abound in the mesenteric folds, and a highly-vas- 

 cular gland of this system, the spleen, is always (except in 

 Amphioxus, Myxine, and the Leptocephalidm) developed in 

 close proximity to the stomach. A pancreatic gland very 

 generally pours its secretion into the anterior end of the intes- 

 tine. 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 ver- 

 tebrate animals, which are most characteristic and peculiar, 

 are the liver and the teeth. 



The Liver. — In invertebrate animals this organ is always 

 ultimately resolvable into caecal 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 proportional 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 in- 

 directly, into a receptacle, the gall-bladder. Amphioxus stands 

 alone among vertebrated animals, in having a csecal diverticu- 

 lum of the intestine for a liver. 



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



