CHAPTEE XXI. 

 ANIMALS WITH A BACKBONE. 



In order to understand how a backboned animal differs 

 from any of the animals which we have thus far studied, we 

 should carefully examine and dissect a fish. 



We have seen that starfish, sea-urchins, lobsters, or in- 

 sects have the body protected by a shell or crust, which 

 protects the muscles and other soft parts within; but now 

 we come to animals which have an internal bony support or 

 skeleton. This consists, besides the skull and limb-bones, 

 of a backbone, which is called the vertebral column, be- 

 cause it is composed of vertebrse. 



Examining the fish, we see that the body is bUaterally 

 symmetrical, i.e., each half repeats the other. There are 

 two eyes, two internal ears, and two nasal openings. The 

 body is protected by scales and is moved by fins. These 

 are either arranged in pairs or are single. The fore pair are 

 called "pectoral " fins, and are attached to the base of the 

 skull. They correspond to the fore-limbs of a beast, or to 

 our own arms; while the pair behind are called the " ven- 

 tral " fins, and they correspond to, or are homologous with, 

 the horse^s hind legs or to our own legs. The single, or un- 

 paired, fins are the "dorsal," "anal," and "caudal" fins 

 (Fig. 137, A). The fins are supported by fin-rays, which 

 are attached to the processes of the vertebral column, as in- 

 dicated in Fig. 138, 7i, g, Ic. 



In front of the base of each pectoral fin is the gill-open- 

 ing; in order to look into it we have to lift up the gill-cover 

 or operculum. Opening wide the gill-opening, and at the 

 same time opening the mouth, we can see the four red gill- 

 arches, supporting the gills. Now, opening the moutt 

 9 



