GENERAL CHARACTERS OF BATRACHIA. 575 



ossified skeleton there are considerable differences in the struc- 

 ture of the vertebral column ; some species having the bodies of 

 the vertebrae concave on both surfaces, as in Fishes ; whilst in 

 the highest forms the vertebras are united by a sort of ball-and- 

 socket joint. The skull, which is broad and flattened, is dis- 

 tinguished from that of Reptiles by its possessing two condyles 

 for articulating with the anterior extremity of the vertebral 

 column ; the orbits are of very large size, and the jaws are 

 usually furnished with small teeth. Another peculiarity of the 

 skeleton is, that the ribs are entirely wanting ; but the vertebra 

 generally give rise to long transverse processes, which seem to 

 a certain extent to take the place of ribs. 



517. The skin is almost always smooth and naked; but it 

 frequently contains a number of glandular organs, from which a 

 peculiar acrid and disagreeable fluid is secreted. It lies loosely 

 about the body, and is covered with a thin epidermis. In a few 

 species it is covered with scales, and these in some cases appear 

 to be analogous to those of Fishes. 



518. In the structure of the nervous system the Batrachia 

 approach the Reptiles, and the cerebral hemispheres usually 

 constitute the greater part of the brain. The organs of the 

 senses are generally more imperfectly developed than in the 

 preceding class. The eyes, however, in the highest species are 

 well-formed, and furnished, as in Reptiles, with eyelids and a 

 nictitating membrane ; but in some of the others they become 

 quite rudimentary, and are even concealed beneath the skin. The 

 ears, also, are usually concealed ; but in the Frogs they have an 

 external opening, furnished with a tympanic membrane ; and the 

 labyrinth consists of three semicircular canals, and a sac filled 

 with little calcareous crystals. The tongue is usually large and 

 fleshy ; but in the Frogs and Toads it exhibits a singular pecu- 

 liarity ; it is fixed to the front of the jaw, whilst its tip, which 

 is free, is turned backwards in the mouth. It can, however, be 

 darted out with the rapidity of lightning, and it is in this way 

 that these creatures obtain their food, which consists, like that of 

 the Chameleon ( 492), in \yhich a somewhat similar organisa- 

 tion prevails, of living Insects. 



