176 



VERTEBRATE ZOOLOGY 



toward dessication, the differences in visibility through water and air, 

 and the differences in conduction of sound waves. In the true ter- 

 restrial forms (reptiles, birds and mammals) special adaptations for 

 making possible embryonic development in the air have been ac- 

 quired, but not so in the Amphibia, which develop through to the 



Fig. 102. — To illustrate the change from fin- type of appendage to the foot- 

 type, and the reverse or secondary adaptation of the foot^type into the fin-type. 



The upper figures (A and B) represent the theoretic mode of metamorphosis of 

 the lobe-fin of the Crossopterygian fish (A) into the foot of the amphibian (B) 

 through the loss of the dermal fringe border and rearrangement of the cartilag- 

 inous supports of the lobe. C, D, E, show the skeletal support of the two types of 

 fin; C, the lobe-fin with the fin-fringe; D, the lobe-fin without the fringe; and E, 

 the foot-stage as seen in an early Carboniferous amphibian. F, G, H, show the 

 secondary reversed evolution of the five-rayed limb (F) of a land reptile into the 

 fin or paddle of an ichthyosaur (G, H). (Redrawn after Osborn.) 



adult condition in the water, undergoing a rather sudden metamor- 

 phosis from the aquatic to the terrestrial physiology. These various 

 primary and secondary changes in structure will, when hsted, serve 

 to indicate the differences between the Fishes and the Amphibia:— 

 1. Respiration. — If we go back to the lobe-finned ganoids for the 

 ancestors of the Amphibia, we find a double respiratory system, 

 branchial and pulmonary, with the pulmonary playing an accessory 

 or secondary r61e. In times of extreme drought or extreme foulness 

 of the water the branchial respiration was held in abeyance and the 

 pulmonary used almost exclusively. The branchial respiration func- 

 tions entirely in early life and it is only with assumption of adult 



