CHARACTERISTICS OF MARINE FISHES 



153 



entirely (Fig. 10-3). Then, there are many catfish, both in fresh and salt water, 

 that are naked. However, these fishes have relatives in South America (Loricari- 

 idae), many of which are fully armored with bony plates and are sometimes re- 

 ferred to as "hard-shelled catfish". 



Sea horses and pipefish, also, have a sort of exoskeleton, composed of bony 

 rings, which allow some flexibility. However, the trunkfish or boxfish have a more 



(Courtesy U. S. Fish and Wildlife Service) 



Fig. 10-3. The common sturgeon {Acipenser sturio). 



complete armor than the species already mentioned, for their "trunk" encases the 

 entire animal, except the tail. The trunk or box is composed of hard, thoroughly 

 united bony plates, which allow no flexibility. Furthermore, the trunk is generally 

 provided with several large spines. One species, Lactophrys trigonus, has a strong, 

 hornlike spine in front of each eye, from which the popular name "cowfish" 

 derives (Fig. 10-4). 



Sharks, with few exceptions, have a rough skin, parts of which formerly were 

 used as an abrasive, called "shagreen". This perhaps is the oddest of all fish 



(Courtesy U. S. Fish and Wildlife Service) 



Fig. 10-4. The trunk fish or cow fish (Ostracion quadricornis) . 



coverings, as the roughness is caused by minute "teeth" set in the skin. These 

 small projections are known to the ichthyologist as denticles, which is a proper 

 designation as structurally they are actually teeth. This, then, is an example of an 

 animal virtually covered with teeth. 



Skates and rays, though related to the sharks, are wholly or partly naked and 

 do not have denticles set in the skin. Some of the skates and rays have bony spines 

 or bucklers, or both, generally arranged in rows on the back, though in some 

 species they are in patches. The porcupine fish is an outstanding example of fish 

 almost completely covered with long and sharp bony spines, resembling the quills 

 of the mammal from which it derives its common name (Fig. 10-5) . 



Scales, of course, are by far the most common covering. They vary so much in 



