710 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the former the exchange of gases takes place, of course, by the outer 

 surface, so that the two functions of respiration and nutrition are 

 relegated to two different portions of the placenta, while in mammals 

 they both take place on the outer surface of the organ. 



Permanence of larval conditions in Amphibia.* — As a general 

 rule the Amphibia when mature cease to breathe by means of gills ; the 

 latter disappear and respiration is carried on solely by means of the 

 lungs. There are, however, a number of cases known, and they are 

 increasing daily, where branchial respiration is carried on for a 

 longer or shorter period of the life of the adult. L. Camerano has 

 lately paid some attention to this subject, and has investigated a 

 certain number of these Amphibians, paying special attention to the 

 dimensions of the adult animal, its organs of reproduction, colour, 

 alimentary system, lungs, and nervous system. The period during 

 which branchial respiration continues varies in different Am- 

 phibia ; the shortest known to him is in Salamandra atra, the 

 longest in Proteus anguineus, the axolotl, and Triton. In almost all 

 the Amphibia of Europe cases are known of an abnormally short or 

 abnormally long " branchiate-period." These may be divided into 

 two classes : (1) those instances of simple hibernation, where the 

 animal has not had time in a single summer to attain maturity ; and 

 (2) other cases where the branchiae remain functional for several years. 

 In this respect, however, the Urodela differ from the Anura ; the 

 former are influenced by local conditions, such as food, presence of 

 floods, &c., which render it necessary for them to continue an aquatio 

 life though the development of the other organs of the body goes on 

 quite as rapidly as in individuals that have adopted a terrestrial life. 

 In the Anura, on the other hand, the permanence of the branchiae for 

 several years is accompanied by an incomplete development of other 

 structures. Such cases are, however, rare, and are not, as in the 

 Urodela, a modification owing to local causes, but are a reversion to 

 an ancestral condition. 



The Amphibia as a class are clearly most nearly related to the 

 fish, and the occasional permanence of a branchiate condition is the 

 best proof of this relationship ; it is, however, none the less possible 

 that branchiae were acquired later, and that the Amphibia were primi- 

 tively land-dwellers, assuming the branchiate condition as a " retrograde 

 metamorphosis " by the adoption of an aquatic life. Keeping in view 

 this possibility it is easy to understand how by artificial interference 

 with the biological conditions, the Amphibia may pass from a 

 branchiate to a pulmonate respiration and back again. 



The old division of Perennibranchiata and Caducibranchiata is 

 therefore unphilosophical : the real proof of the adult condition of an 

 Amphibian is the maturity of the reproductive organs, and its branchi- 

 ate or pulmonate condition must be neglected since it is merely an 

 instance of dimorphism dependent upon the influence of the environ- 

 ment. 



* Mem. E. Accad. Sci. Torino, xxxv. (1883) 64 pp. (2 pis.). Natuiforscher, 

 xvii. (1884) pp. 273-4. 



