CLASSES OF ANIMALS 



455 



The eyes are similar to those of other vertebrates, but are 

 generally very shortsighted. There are two nostrils, but these 

 do not connect with the food tube, as they do in all the other 

 classes of vertebrates, but end in odor sacs. There is no external 

 ear, but the internal ear somewhat resembles that of the other 

 vertebrates, including the semicircular canals, or balancing or- 

 gans. In addition the "lateral line," which is easily distin- 

 guished in most of the familiar fishes, is well supplied with 

 nerves and joins the brain close to the ear nerves. It is 

 probably sensitive to 

 certain vibrations in 

 the water. 



In common with 

 all other vertebrates, 

 fishes have blood con- 

 sisting of plasma and 

 white and red cor- 

 puscles. The blood is 

 contained within the 

 blood vessels, and the 



circulation is maintained by means of a two-chambered heart. 

 This is in contrast to the three-chambered heart of the batra- 

 chians (see page 87) and the four-chambered heart of birds 

 and mammals. 



There are over 15,000 species of fish known ; some 3000 species 

 live in the waters of the United States. A very large proportion 

 of these are fit for food, although not all of them are extensively 

 used. Modern methods of preserving and packing food have 

 made it possible to extend the use of marine fish to inland com- 

 munities, and of seasonal fish to all parts of the year, and so on. 

 In addition to the use of fish as food many species yield valu- 

 able oils ; the cod and menhaden are the most important. The 

 skin of some of the sharks is w^orked over into a hard, tough 

 leather called shagreen, which is valuable for buffing and 

 polishing. The eggs of sturgeons and a few other species are 

 preserved in large quantities and eaten as caviar, which is 



Fig. 187. Lateral line in brook trout 



The line running along each side from the gill cover 

 to the tail is made up of nerve end-organs that are 

 sensitive to certain kinds of vibrations in the water 



