CHAPTER 10 



Characteristics of Marine Fishes 



Samuel F. Hildebrand * 



Formerly Ichthyologist, U. S. Fish and Wildlife Service 



Definition 



Many kinds of animals that have httle in common, except a water habitat, are 

 called fish. Some of these are very low forms of animal life, as for example jelly- 

 fish, which are composed of a jellylike mass and have no hard (bony) parts. The 

 sponges, which are also a low form of animal life, are generally listed with the 

 fishes in statistical reports and the means of procuring them is referred to as 

 the "sponge fishery". Shellfish consist of both moUusks and crustaceans: the former 

 are mostly oysters, clams, and scallops; the latter includes shrimps, crabs, lobsters, 

 and crawfish. Structurally these forms are all far removed from those that provide 

 the basis for the accounts that follow. Whales and porpoises are an exception to 

 this definition: they are not true fishes but warm-blooded mammals. 



These animals are commonly referred to in technical works as fish and fishlike 

 vertebrates. They may be defined as cold-blooded animals, having a backbone 

 (which is cartilaginous in the lower groups) and, with few exceptions, limbs or 

 rudiments of limbs represented by fins. They live in water, wherein they breathe 

 by means of gills. Although the skin may be naked, it is usually covered with 

 scales, denticles, bony plates, spines, or tubercles. 



The lowest forms of animals considered in these pages are not fish, but only 

 fishlike animals. They are the lancelets, Leptocardii, and the lampreys, Cyclostomi. 

 That great group of animals, which meets all the requirements of the definition of 

 a fish offered in the preceding paragraph, may be divided into two main groups: 

 namely, the sharks, skates and rays, and chimaeras, Selachii, and so-called true 

 fishes, Pisces. The Selachii have cartilaginous skeletons, whereas the Pisces, with 

 few exceptions, have bony skeletons. These several groups of fish and fishlike 

 animals will be discussed in more detail subsequently. 



The Antiquity of Fishes 



The first animal vdth a backbone was a fish. Its ancestor, it is believed, was a 

 marine creature that incased its spinal cord in a hard protective covering, which 

 gave its body rigidity and suppleness. A group of small marine animals exists to- 

 day that probably resembles rather closely this primitive ancestor, which lived 

 many millions of years ago. These small living animals, which attain a length of 

 only a few inches, are the lancelets, also known as amphioxus. These are listed in 

 the classification of fishes and fishlike animals as the Leptocardii. The lancelets 



* Deceased. 



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