THE VERTEBRATES 419 



two pairs of appendages for locomotion (fins in fishes, fore 

 and hind legs in amphibians and many higher forms, wings 

 and legs in birds, arms and legs in man) ; (6) red corpuscles 

 in the blood. Review your study of the frog and compare 

 with this list of important points of structure. 



If we had time for dissection and careful study of a fish, 

 a salamander, a snake, a turtle, a bird, and a mammal, we 

 should find that they have the same organs and arranged in 

 the same relative positions as those in the frog. Such com- 

 parative study must be left for advanced courses in colleges. 



FISHES * 



345. External Structure of a Fish. (L) Any available fish from 

 the market should be examined. Note the two pairs of lateral 

 fins ; and the dorsal and ventral fins on the median line. Compare 

 several species of fishes, or pictures of them, as to the arrangement 

 of the fins ; and especially note that the posterior pair of fins is in 

 some fishes near the anterior pair. Note arrangement of scales. 



Examine the mouth and gills. 



Observe (1) the movements of the body and fins, and (2) the 

 mouth and gills in living fishes in an aquarium. 



346. Distribution and Habits of Fishes. Of the more 

 than 13,000 species of fishes now known to exist, about one- 

 fourth are found in or near North America. 



A remarkable fact regarding the distribution of fishes is 

 that certain species live at various depths in the oceans down 

 to over 2000 fathoms. At this depth there is complete dark- 

 ness, a constant low temperature, quiet water, and the 

 enormous pressure of over 5000 pounds per square inch of 

 skin. To such conditions the deep-sea fishes are adapted. 



* The singular "fish" is commonly used for singular and plural when 

 referring to one or more specimens of a single species of fish, e.g., we may 

 say correctly "a ton of common codfish" which might include hundreds of 

 individuals of the common species. The plural word " fishes " refers to more 

 than one species, e.g., "there are at least 140 species of codfishes among the 

 more than 13,000 different species of fishes now known." 



