Chapter U 



ADAPTATIONS TO MEDIA AND SUBSTRATES 



(A) Aquatic Vision 



Definition — Before we consider the requirements and consequences of 

 seeing through water we need to decide what we mean by aquatic in this 

 connection. There is no doubt that all but a few jfishes are aquatic; but 

 one may read in one book that the seal is an aquatic mammal, and in 

 another that he is an amphibious vertebrate. We may have to be a little 

 arbitrary about our definition of 'aquatic', arriving at it by a process of 

 elimination, and justifying our arbitrariness only in a later section. 



The Amphibia (amphi = hoth., 6/05 = life) were given their name be- 

 cause they spend part of their lives in water and part on land. The word 

 amphibian means a member of the Class Amphibia. It is sometimes used 

 as an adjective, but should be avoided in favor of amphibious. This word 

 is much older than the scientific term Amphibia, and does not really 

 connote the same thing at all. Amphibious animals are those which are 

 in and out of the water off and on as an everyday thing, and 'equally 

 at home' in both media. Very few members of the Amphibia behave at 

 all in this way. Most of the common frogs (family Ranidae) do. The 

 less familiar but much more numerous tree-frogs, toads, and land sala- 

 manders do not. Most amphibians, then, are not amphibious. Rather, 

 they are aquatic for a part of their life-cycles (as tadpoles) and terres- 

 trial for the remainder with only brief annual visits to water to breed. 

 Some salamanders and a few anurans (e.g., Pipa and Xenopus spp., 

 Telmdtobius micro phthalmus) never leave the water — they are as aquatic 

 as any fish. A few anurans (e.g., Hyla zeteki) never enter ponds or 

 streams, their eggs developing in mere spoonfuls of water between the 

 leaves, or in the central core, of bromeliad plants; and these forms are 

 as terrestrial as a human — whose embryo, inside the amnion, also floats 

 in water. The salamanders Hydromantes italicus and Oedipus adspersus 

 are thoroughly terrestrial, and give birth to their young. 



We shall consider as aquatic, then, those vertebrates which never leave 

 the water. These animals with strictly aquatic vision include nearly all 

 fishes, some amphibians, the sea-cows, and the whales. It so happens that 



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