362 METAZOAN PHYLA 



fins become replaced by jointed limbs (Fig. 246) which form a system of 

 levers, these limbs being divided into three portions — upper limb, 

 lower limb, and a third portion which in the forelimb becomes the carpus 

 and the forefoot, and in the hind limb the tarsus and hind foot. A tail 

 fin is present in some amphibians, but this fin is simply a fold of the skin 

 without a fin skeleton. In order better to support the weight of the 

 body, the limb skeletons become rather firmly attached to the axial 

 skeleton and the skeleton as a whole becomes to a greater extent bony 

 and distinctly more rigid. 



A fifth adaptation in the amphibians for terrestrial life is seen in the 

 eyes, which become supplied with lids for protection and with lachrymal 

 glands to moisten the eyeball and prevent it from drying. The lens 

 also becomes more flattened and capable of more distant vision. 



Since sound waves are transmitted less perfectly in the air than 

 in the water the ear also shows adaptation. A middle ear is formed 

 which in many amphibians is closed externally by a tympanic membrane 

 and across which sound is transmitted by means of a bony rod called the 

 columella. The columella is articulated with a second bone, the stapes, 

 set into the opening in the wall of the sacculus. A eustachian tube is 

 developed connecting the middle ear with the pharynx. This tube and 

 the cavity of the middle ear together represent a modified pharyngeal slit 

 and correspond to the spiracular canal of the elasmobranchs. 



391. Origin of Terrestrial Adaptations. — Although, as has been 

 noted before, certain fishes do at times leave the water it is only for a 

 brief interval that they do so and they show no changes adapting them 

 to life in the air. Other fishes such as the lungfishes have acquired 

 adaptations which enable them to breathe air when the water becomes 

 very foul or during periods when they remain dormant in the mud left 

 by the drying up of bodies of water. These adaptations, however, 

 are not believed to be exactly similar to the pulmonary adaptations of 

 Amphibia. 



For the origin of the amphibians one must go back to the Crossop- 

 terygii, which were noted as possessing larvae similar to amphibian 

 tadpoles and which used the forelimb as a means of support while resting 

 on the bottom. The Devonian period (Fig. 312) was the age of fishes, 

 at which time they were the highest animal types living. It is thought 

 that during the latter part of this epoch there were seasons of warmth 

 and heavy rainfall followed by more and more prolonged periods of 

 drouth. Under these conditions it is beheved that from the lobe- 

 finned ganoids arose different types of animals showing adaptations 

 to terrestrial life and that the amphibians represent a successful type 

 which has persisted to the present day. The earliest trace of amphibians 

 is the footprint of a three-toed animal found in Pennsylvania in rocks 

 of the upper Devonian period. This animal possessed feet instead of 



