30 LABOR^'tyORY MANUAL FOR VERTEBRATE ANATOMY 



mouth cavity. This arrangement permits the animal to breathe without opening the mouth, 

 a decided advantage in air-breathing animals, since thereby the drying of the mouth cavity is 

 avoided. In the lower vertebrates an internal ear only is present. To this is added, begin- 

 ning with the anuran Amphibia, a middle ear, closed externally by the tympanic membrane. 

 In Anura and many reptiles the tympanic membrane is level with the surface of the head, but 

 in some reptiles it begins to sink below the surface. In birds and mammals it has descended 

 deeply into the head, forming a narrow passage, the external auditory meatus. Around the 

 external rim of the meatus in mammals the skin elevates to form a sound-catching device, the 

 pinna. Pinna and meatus constitute the external ear. 



6. The gill slits and gills present in the fishes and lower Amphibia disappear in the adults 

 of the higher Amphibia and all forms above them. This is due to the assumption of the air- 

 breathing habit. 



7. The trunk bears two pairs of appendages. These are fins in fishes, but become limbs 

 in all vertebrates above fishes. Stages in this transformation are very imperfectly known. 

 The parts of the limbs are the same through all of the vertebrates, although they are subject 

 to considerable modification. The most primitive limbs, in structure, form, and position with 

 reference to the body occur in the urodele Amphibia. In higher vertebrates the position of the 

 limbs is altered by bending and torsion, resulting in an elevation of the body above the ground 

 with a correspondingly more rapid progression. As a still further aid to rapid movement the 

 digitigrade or unguligrade mode of walking has been adopted in many cases. Loss of digits 

 is quite common among vertebrates; the missing digits are nearly always the first or last ones, 

 rarely the middle ones. 



8. In nearly all vertebrates except mammals the intestine and the urogenital ducts open 

 into a common chamber, the cloaca, which communicates with the exterior by a single opening, 

 the anus or cloacal aperture. In all placental mammals the intestine and the urogenital system 

 open by separate apertures, the urogenital opening being situated always anterior to the anus. 

 The term anus, therefore, does not have the same significance in mammals as in other 

 vertebrates. 



