CHAPTER FORTY-FIVE 



Definition 

 of a fish 



FISHES 



1. FISHES may be briefly defined as aquatic verte- 

 brates in which the skull is well developed, with jaws, and 

 the pectoral and pelvic girdles are developed, each usu- 

 ally supporting a pair of fins. Respiration is by means 

 of gills. Fishes exist in the sea and in fresh waters 

 throughout the world. Although the remoter ancestors 

 of fishes were undoubtedly marine, there is some reason 

 for thinking that the actual evolution of the first fishes 

 was in fresh water. Even sharks, now characteristically 

 marine, appear to have formerly lived in fresh water - 

 as shown, for example, by their occurrence in the nod- 

 ules of Pennsylvanian age at Mazon Creek, Illinois. 

 These were primitive types, little resembling the mod- 

 ern sharks in the details of their structure ; but there is 

 a typical shark existing today in the fresh water of Lake 

 Nicaragua, Central America. 



2. The classification of fishes has given rise to many 

 tionoffishes Differences o f opinion, and is still subject to modifica- 

 tion. The object sought is to arrange all the fishes in 

 accordance with their natural relationships, assuming 

 them to have evolved from a common ancestor. While 

 there must be a true or ideal classification, accurately 

 representing the historical facts, it is hardly to be ex- 

 pected that we shall ever completely attain it, though 

 we continually move toward it. Regarding the fishes 

 (Pisces) as a class, we have the following principal divi- 

 sions : 



(a) Subclass Elasmobranchii, or the sharks and their 

 relatives. Some would separate these as a class 

 distinct from the Pisces. In the true sharks 

 and rays (or skates) - - the latter being broad, 



342 



Classifica- 



Sharks and 

 rays 



