i!34 ' VERTEBRATES. 



monly softer and less earthy than those of other animals, and in 

 some they are entirely cartilaginous. The stomach and intestines 



Skeleton of Perch, showing the Bony Structure. 



are formed in a manner similar to those of other vertebrates, and 

 digestion is conducted in the same general way. Their food con- 

 sists principally of other fish, worms, and shell-fish. 



Most fish possess a singular organ called the "swimming- 

 bladder." This is a membranous pouch, varying exceedingly in 

 size and shape, situated close under the spine, and filled by some 

 means with gas, mostly found to be nitrogen, but in deep-sea-fishes, 

 an excess of oxygen is discovered to exist. The fish seems to be 

 able to rise or sink by means of compressing or expanding this 

 pouch, without being forced to make use of its tail or fins. 



The smooth scaly covering with which most fish are fur- 

 nished, is admirably fitted both for defence against the water, and 

 for enabling the fish to glide easily through places where a rough 

 covering would have held it prisoner. Many valuable character- 

 istics are derived from the shape of the scales in difiierent fish. 

 There are four principal varieties, called, 1. Placoid, or flat scales; 

 2. Granoid, or polished Scales ; 3. Ctenoid, or toothed scales ; and 

 4. Cycloid, or circular scales. 



I. BONY FISHES. 



The Common Perch is well known to anglers both as a 

 " bold biting fish," and as a fish that does not yield up its life 

 without endangering the person of its captor ; for the formidable 



