AMPHIBIA 179 



an amphibian hand. The footprint of Thinopus, the earliest am- 

 phibian trace, does not reveal any of the skeletal parts, but it is 

 likely that only two fingers were fully developed and two others 

 partially separated. In the development of the amphibian foot 

 as shown in Fig. 104 the three-fingered condition persists till rather 

 late in development; then a fourth finger appears well down on 

 the ulnar side of the hand and a rudiment of the fifth (the little 

 finger) appears as a mere bump. The thumb, index, and second 

 finger seem to be phylogenetically the oldest digits, and this is im- 

 portant in connection with the loss of fingers in other vertebrates; for 

 it is always the latest to develop that is first lost. With the substi- 

 tution of a foot for a fin the kind of movement becomes profoundly 

 altered. Instead of a mere paddling back and forth, a variety of move- 

 ments are necessary and thus the old myotomic musculature becomes 

 decidedly modified until nearly all traces of the segmental arrange- 

 ment of muscles are lost. 



4. Changes Due to Increased Specific Gravity. — ^When the ani- 

 mal comes from the water to the land it is relatively heavier; hence 

 there is need of a more rigid skeleton, stronger Umb girdles and limb 

 skeleton. This is accomplished by more complete ossification of the 

 parts of the skeleton that bear the most weight. There is likely also 

 to be a reduction of dead weight, such as exoskeletal parts. In modern 

 Amphibia the exoskeleton has disappeared completely (except in caecil- 

 ians, where it is rudimentary), but in the stegocephalians the head 

 armor persisted, while that of the rest of the body largely disappeared. 



5. Responses to Seasonal Changes of Temperature are much more 

 necessary on land than on water. Nearly all Amphibia hibernate 

 during winter either by burying themselves in the earth or in water. 



6. Changes for Avoiding Dessication. — If a fish is taken out of 

 water it soon dries up on the surface and becomes stiff. Amphibia, 

 however, have abundant skin glands, secreting moist mucus which 

 keeps the skin in proper condition to perform a respiratory function 

 and helps it to retain its flexibihty. Some of the Amphibia have 

 rudimentary lungs and respire almost exclusively through the skin. 



7. The eyes change, especially in the shape of the lens, which 

 becomes flattened instead of spherical. 



8. An external sound receptor appears in the form of a tympanic 

 membrane, which is in communication with the inner ear through a 

 columella, a bony apparatus that vibrates with the ear drum. 



