THE HEART IN THE AMPHIBIA. 183 



bones. In existing Amphibia their structure is simple, but 

 in tlie Labyi-iutliodonts, the parietes of the teeth, at a 

 certain distance below the summit, become longitudinally 

 folded, and each fold may be again longitudinally plaited, 

 so that the transverse section of the tooth acquires a very 

 complicated structui-e, the pulp-cavity being subdivided 

 into a great many radiating and branching segments. The 

 structure is similar in principle to that exhibited by the 

 teeth of many of the Ganoidei. In many of the Labyrrn- 

 thodonts, again, two of the anterior mandibular teeth take 

 on the form of long tusks, which are received into fossae, or 

 foramina, of the upper jaw, as in most existing CrococUlia. 

 The tongue is fixed to the floor of the mouth in Urodela and 

 Gymnopliiona, and remains undeveloped in the genera 

 Pipa and Dactylethra, which have thence been termed 

 Aglossa. In other Batrachia, the tongue, which is usually 

 long, and fixed by its anterior end to the symphysis of the 

 mandible, can be rapidly protruded and used as an organ 

 of prehension. No distinct salivary glands have been ob- 

 served in the Amphibia. Many male Batrachia have the 

 mucous membrane of the floor of the mouth produced into 

 pouches which can be distended with air. 



The simple alimentary canal is usually short, and much 

 longer in the larvae (which are vegetable feeders) than in 

 the adults. A gall-bladder is always present. 



The heart presents two auricles, a single ventricle and a 

 bulbus arteriosus. A venous sinus, the walls of which are 

 rhythmically contractile, receives the venous blood from the 

 body, and opens into the right auricle. In Proteus, Menobran- 

 chus, and Siren, the septum of the auricles is less complete 

 than in the other Amphibia. The left auricle is much smaller 

 than the right, and a single pulmonary vein opens into it. 

 The interior of the ventricle is more like a sponge than a 

 chamber with well-defined parietes. The walls of the long 

 bulbus arteriosus contain striated muscular fibres, and are 

 rhythmically contractile. Yalves are sometimes placed at 

 each end of it, and it may be imperfectly divided into two 



