THE ALIMENTARY CANAL. 201 



in a single row; sometimes they are fastened to the upper edge 

 (acrodont), sometimes to an external, strongly projecting ledge 

 of the flat 'dental groove (pleurodont), rarely, as in Crocodiles, 

 they are wedged into special alveoli. Hooked teeth may also be 

 present on the palatine and pterygoid bones, and in this case 

 they frequently (e.g., in non-poisonous Snakes) form an inner 

 arched row on the roof of the mouth. In the poisonous Snakes 

 special teeth of the upper jaw are traversed by a groove or canal, 

 and enter into clcse relation with the ducts of poison glands, the 

 secretion of which passes into the wound through the groove or 

 canal in the poison teeth. Salivary glands are found in Snakes and 

 Lizards, both in the lips and on the lower jaw, and a sublingual 

 gland may also be present. The possession of the latter is charac- 

 teristic of the Chelonia. 



The oesophagus is very long, and is capable of an extraordinary 

 degree of dilatation. Its walls are usually folded longitudinally, but 

 they may also be beset with large papillae, as in the Turtles. The 

 stomach is usually arranged longitudinally, except in the Chelonia r 

 which possess like the Frogs a transversely-placed stomach. The 

 stomach of the Crocodiles resembles that of Birds, both by it& 

 rounded form and by the strength of its muscular walls. The small 

 intestine is but little coiled, and remains relatively short; in the 

 Land-Tortoises^ alone, which live on vegetable matter, is the intestine 

 more than six or eight times longer than the body. The broad large 

 intestine (rectum) usually begins with an annular valve, and often 

 with a csecum. and leads into the cloaca, which opens beneath the 

 root of the tail by a round opening, or as in the Snakes and Lizards 

 by a transverse slit (Plagiotretna). Liver and pancreas are never 

 absent. 



The Rtptilia breathe exclusively by lungs, which have the form of 

 spacious sacs, with alveolar projections of the walls, or 'with wide 

 spongy cavities (Tortoises and Crocodiles). In the Snakes and snake- 

 like Lizards the lung on one side is more or less reduced, while the 

 other obtains a correspondingly greater size. The posterior end of 

 the latter loses not only the cellular alveolar spaces, but also the 

 respiratory vessels, and has the form of an air-reservoir (foreshadow- 

 ing the air-sacs of Birds), which renders respiration possible dur- 

 ing the slow process of swallowing. The afferent air-passages are 

 always differentiated into a larynx beginning with a slit-like glottis, 

 and into a long trachea and bronchial tubes, supported by carti- 

 laginous and often bony rings. A membranous or cartilaginous 



