INTRODUCTORY 233 



For example, there are aquatic insects of many kinds, and 

 members of the class Amphibia such as newts and frogs, which 

 either temporarily or permanently live in lakes, ponds, or rivers. 

 Many of these, like whales and porpoises, are still dependent 

 on the air for respiration, they are entirely unable to obtain the 

 oxygen which is necessary for life from the water, and must 

 therefore rise to the surface at frequent intervals, and none of 

 them can move in the water with the grace and agility of the 

 fishes. 



Fishes may be generally defined as vertebrate animals 

 which breathe in water by means of gills situated on the sides 

 of slits or clefts leading from the cavity of the throat to the 

 surface of the skin, and which possess paired and unpaired fins 

 supported by fin-rays. If we consider the true or typical fishes 

 we may add to the definition the possession not merely of a 

 backbone or vertebral column but also of a skull, and not only 

 of a skull but of distinct jaws placed transversely to the length 

 of the body and hinged at the angles. The Lampreys and 

 their allies resemble fishes in their breathing organs and in the 

 possession of a skull, but they have no transverse jaws, the 

 mouth being circular and suctorial, and no paired fins. The 

 Amphioxus or Lancelet differs still more in the absence of jaws, 

 fin-rays, vertebrae, and skull. Fishes are in many respects 

 similar to Amphibia, especially when we consider the lung- 

 fishes or Dipnoi which breathe partly by means of lungs, and 

 certain Amphibia which retain gill-slits throughout life. The 

 only character which separates all Amphibia without exception 

 from all fishes is the absence of fin-rays in the former. 



The structure of fishes must now be considered in greater 

 detail. In general the more primitive condition of the organs 

 is found in the sharks, dog-fishes, and rays (Elasmobranchii), 

 and the common small ground dog-fish, Scyllium canicula, is a 

 good example of such fishes, while the gold-fish or perch is 

 equally typical of the scaly and bony fishes ( Teleostei). The first 

 part of the skeleton to be developed is a cylindrical unjointed 

 rod of gelatinous tissue which runs through the axis of the body, 

 and is technically termed the notochord. The formation of this 

 structure is the first step in the development of the skeleton in 

 the embryo fish, or indeed in the embryo of any vertebrate, and 

 the segment of the spinal column or " vertebrae " are developed 



