136 FIRST LESSONS IN ZOOLOGT. 



fishes, such as the perch, common sun-fish, chub, roach, etc., 

 are highly colored while laying their eggs, and thus the sexes 

 are mutually attracted, in the one case by music and in the 

 other by bright colors. Finally, the sounds of fishes may 

 be compared with those of reptiles, birds, and mammals, 

 the air-bladder corresponding to the lungs of the higher 

 Vertebrates, while the pneumatic duct is comparable with 

 the windpipe of birds and mammals. 



In swimming, the propelling motion is mainly exerted by 

 the tail, the movements of which are somewhat like those 

 of an oar in sculling. The spines of the tail-fin are mov- 

 able, and are capable of being brought into such a position 

 that the fin will' meet with less resistance from the water 

 while the tail is bent than straightened, and it is when be- 

 ing straightened that the fish is propelled. The move- 

 ments of the pectoral and ventral fins are to steady the fish 

 and to elevate and depress it, while the dorsal and anal fins 

 steady the body and keep it upright, like a dorsal and ven- 

 tral keel. 



Having studied a fish, the student can now understand 

 what a vertebrate is and perceive the great differences be- 

 tween vertebrates and the lower or invertebrate animals. 

 And, before we go farther, we can, since everybody is famil- 

 iar with the higher backboned animals, such as the frog, 

 lizard, bird, or beast, pass in review the chief distinguishing 

 marks of a vertebrate. 



Vertebrates in general have bodies which are symmetri- 

 cal, i.e., the two sides repeat each other; they have a brain- 

 box or skull, containing the brain and the mouth and phar- 

 ynx, with two eyes, two ears, and usually two nasal open- 

 ings. To the trunk are attached two pairs of limbs; the 

 arms in man corresponding to tlie fore-legs of the horse or 

 dog. 



Now, if we cut a fish in two, and closely examine the sec- 

 tion, we shall notice that above the backbone is a little cav- 

 ity containing the nervous cord, and below a much larger 

 cavity containing the viscera, i.e., heart, liver, stomach, or 



