ALIMENTARY CANAL. BODY-CAVITY. 277 



ginous plate which is coextensive with the membrana tympani. 

 The shaft of the columella is frequently ossified, its two ends 

 remaining cartilaginous. In the Urodela, in which it exists 

 embedded in the muscles, its stapedial portion may be ossified 

 and it is connected to the quadrate by ligament. It is probably 

 homologous with the hyomandibular of fishes, though in the 

 Amphibia it is never connected with the hyoid arch. The 

 membrana tympani is attached to a cartilaginous ring. 



The cup-like sense organs of the lateral line found in the 

 aquatic forms and in the larvae of land forms have already been 

 referred to (p. 273). 



Alimentary canal. The mouth opening is a wide slit. 

 Teeth, which are ankylosed to the bones, are present upon the 

 premaxillae, maxillae, andvomers, soijaifcimes on the dentaries, 

 palatines, and parasphenoid. They^are absent in Pipa and 

 some toads. The tongue is immovable in Urodeles, absent in 

 Aglossa, movable and free behind in other Anura, in which it is 

 used as a prehensile organ. The posterior n res and eustachian 

 tubes have already been referred to. Salivary glands are not 

 present. In many male Anura the lining of the buccal cavity 

 is produced into sacs, the vocal sacs, which act as resonators. 

 In Rhinoderma they are used as nurseries for the young. Oeso- 

 phagus, stomach, small intestine and rectum are present. The 

 hind end of the rectum is called the cloaca and possesses a median 

 ventral appendage, the bladder.* The urinary and generative 

 ducts open into the cloaca. The cloaca opens to the exterior 

 by the anus. Liver and pancreas are present, and the former 

 has a gall bladder. 



The thyroid, unpaired in its origin, but becoming paired 

 later, is present ; and an organ representing the thymus and 

 derived as epithelial buds from some of the branchial pouches 

 is present close to the angle of the lower jaw (in the Anura behind 

 the tympanic cavity and beneath the depressor mandibulae 

 muscle). 



The body-cavity is completely divided into the pericardial 

 and peritoneal cavities. The peritoneal cavity extends forwards 

 on each side of the pericardial. Abdominal pores are absent. 



* So-called alla^toic bladder, though the Amphibia have no allantois 

 in the embryo. 



