PRACTICAL ESSAY. 83 



is a huge loach-like kind of fish found in continental waters, is a 

 notable exception to this rule, for the male (not the female) watches 

 the eggs most jealously for several weeks. The stickleback builds 

 a nest for its eggs, and guards it well 



PRACTICAL ESSAY. 



This is a fit place for a few observations on the natural history 

 of fishes, or " Ichthyology." 



Vertebrate animals (animals with a backbone) are divided into 

 four classes, of which the last is the great group of Fishes. It is 

 undoubtedly the largest group of all, and is broadly divided into 

 two series: the bony fishes those which have a bony skeleton; 

 and the cartilaginous fishes, whose skeletons are of cartilage or 

 gristle. 



The first series is divided into six orders : 

 Order I. has 15 families. 

 II. 5 

 HI. 3 

 IV. i 

 V. i 

 VI. 2 

 Series II. is divided into two orders : 



Order I. has 2 families. 



II- 2 



These families are further divided into genera and species. 

 Our British fresh-water fishes only extend over a small section of 

 the group thus classified. The annexed table will show at a glance 

 the names and number of such fishes. (See pp. 84, 85.) 



