Classification and Origin of Fishes 



The animal kingdom is divided into major divisions known as phyla. 

 The fishes belong to the great phylum Chordata, which includes all the 

 vertebrates or backboned animals. This phylum is divided into classes, 

 each of which is subdivided into orders. The orders are subdivided into 

 families, which in turn are divided into genera. Each genus contains one 

 or more species. The combination of the generic name and the specific 

 name constitutes the scientific name. To be complete the scientific name 

 should be followed by the name of the author or person who originally 

 described the animal under the species designated. If the genus has since 

 been changed, the name of the original author is given in parentheses. 

 In recent years minor variations of certain species have been noticed in 

 different geographical regions. When these variations occur within defi- 

 nite geographical ranges a third name designating a subspecies is added. 



For many years all fishes were included in one class of vertebrates, 

 under the name Pisces. In recent years zoologists have realized that the 

 fundamental structures of many fishes vary so widely that this group 

 really contains a number of classes. Consequently investigators have 

 developed various classifications, each dividing the fossil fishes and 

 modern, or living, ones into a number of classes. Considerable contro- 

 versy has arisen as to which of the several classifications is best, and 

 consequently no one general classification is in universal use. 



In general, modern, or living, fishes may be separated into three 

 distinct groups. The first are commonly called the cyclostomes and are 

 represented by the lampreys and the hagfishes. The cyclostomes are 

 jawless forms without paired fins. The cartilaginous fishes (Chondrich- 

 thyes) form another group. They have well-developed jaws and paired 

 fins, but their skeletons are entirely of cartilage, without any bone at 

 all. This group includes the sharks, skates, rays, and chimaeras, almost 

 all of which are marine fishes. None are found in the fresh waters of 

 North America except in some waters immediately adjacent to the 

 Gulf of Mexico. The bony fishes (Osteichthyes) form the last group. 

 They are highly developed fishes with skeletons formed more or less 

 of bone. This group includes a vast number of marine fishes and all 

 the species of the fresh-water fishes of North America, with the excep- 

 tion of the few species of lampreys. 



Fishes are said to be the oldest of the backboned, or vertebrate, 

 animals. The oldest well-known vertebrate fossils, remains of animals 

 that lived millions of years ago, are those of fishlike forms known as 

 the Ostracodermi. These earliest forms were without well-developed 

 skeletons, and some were covered with plates. They lacked upper and 



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