30 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



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

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

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

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

 In A n ura and many reptiles the tympanic membrane is level with the surface of the head, b 

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

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

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

 pinna. Pinna and meatus constitute the external ear. 



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

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

 breathing habit. 



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

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

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

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

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

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

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

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

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

 rarely the middle ones. 



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

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

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

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

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

 vertebrates. 



