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CLASSIFICATION OF VERTEBRATES. 



in the anura, enters into the accessory auditory structures, 

 forming an Eustachian tube leading from the tympanic cavity 

 to the pharynx. A stapes is also developed, and in the anura 

 this is joined to a columella (possibly derived from the hyoman- 

 dibular) which stretches across the middle ear to the tym- 

 panic membrane. In the urodeles and caecilians the columella 

 and Eustachian tube are absent, and frequently the stapes 

 articulates directly with the quadrate. 



The oral and pharyngeal cavities are ciliated ; and into them 

 open, in front, the internal nares, and behind, the slit-like glottis, 

 communicating with the more or less elongate trachea. In the 

 young, and in phanerobranchs and derotremes, gill slits occur 

 in the pharyngeal region. Of these, three open to the exterior, 

 while one (or in some cases two) pouches behind these never 

 break through. 



The alimentary tract may be nearly straight in the elongate 

 forms, or be greatly convoluted in those with shorter bodies, 

 the convolutions reaching their extreme in the herbivorous tad- 

 poles of the anura. The rectum is short, and opens into the 

 cloaca. The liver is two-lobed, and in the anura the left lobe is 

 more or less completely sub-divided. The pancreas is flattened 

 and lobulated. 



In the young of all external gills occur, and these may per- 

 sist throughout life (perennibranchs). These gills are ecto- 



clermal in origin, and arise as 

 outgrowths from the side of the 

 neck before the gill slits break 

 through. Usually they are more 

 or less branched and feathered, 

 but in Cazcilia compressicauda 

 they are large sacs. The ento- 

 dermal gills are a later appear- 

 ance, and arise from the walls of 

 the gill clefts. These clefts at 

 first open freely to the exterior ; 

 but in the adults of most they 



become closed, remaining permanently open only in the peren- 

 nibranchs and derotremes. In the anuran tadpole an oper- 



FIG. 277. Head of young Die- 

 myctylus viridescens, showing lateral 

 line openings and remains of gill clefts. 



