4 A HISTORY OF FISHES 



member of the Whale tribe (Cetacea) from a large fish such as 

 a Shark; in the Cetaceans the flukes or lobes of the tail are 

 horizontal, in the fishes they are vertical (Fig. i). It is of some 

 interest to note that Aristotle (384-322 B.C.) was well aware of 

 the differences between fishes and aquatic mammals, whereas 

 many of the writers in historical times classed them all together 

 as fishes. The distinctions between the two groups do not 

 appear to have been generally understood until the later part 

 of the seventeenth century, and ignorance as to the real nature 

 of the Cetaceans must often have led our pious ancestors to 

 break Lent, since they enjoyed steaks and cutlets of Whale, 

 Porpoise, or Seal on fast days under the fond delusion that 

 they were consuming fish ! 



The Ichthyosaurus, an extinct aquatic reptile, exhibits the 

 same general fish-like form and paddle-like limbs, but these 

 have clearly been acquired independently, as in the Whales, 

 as a result of the adoption of a life in the water (Fig. 3). 



There is yet another creature, common in all our ponds and 

 streams during the spring months, often confused with the 

 fishes in the popular mind, namely the tadpole, which is, of 

 course, the young stage of a Frog or Toad. The Newts, Sala- 

 manders, Frogs and Toads belong to a class of vertebrates 

 known as Batrachians or Amphibians, the latter name referring 

 to the fact that they are not only amphibious in the popular 

 sense, living partly in the water and partly on dry land, but 

 are also actually adapted during the early part of their life to 

 breathe under water by means of gills like the fishes, and at 

 a later period to breathe air by means of lungs like the reptiles. 

 But some amphibians never breathe under water at any 

 stage of their existence, not even when immature, and others 

 retain their gills throughout life. How, then, is it possible 

 to distinguish any amphibian from any fish? By the organs of 

 locomotion. In all amphibians the paired limbs are legs in the 

 adult state, in fishes they are fins. 



To summarise, a fish may be defined as a vertebrate adapted 

 for a purely aquatic life, propelling and balancing itself by 

 means of fins, and obtaining oxygen from the water for 

 breathing purposes by means of gills. Fishes, thus defined, 

 were formerly regarded as representing a single class of the 

 great sub-kingdom of vertebrates, a class equivalent to the 

 birds {Aves) or the reptiles [Reptilia) ; but a more thorough 

 knowledge of their anatomy and evolutionary history has led 

 to a different conclusion. The Lampreys and their allies 



