154. BACKBONED ANIMALS. 
parent form for three years. For an illustrated account of nest-building 
fishes see the article on the subject by the author in “ Harper’s Month- 
ly,” Christmas number, 1883. 
Fic. 192.—Figure of a full-grown lamprey and of the young lamprey, for- 
merly called Ammocetes, showing the seven holes through which it takes 
in water to breathe. 
Class II].—THE TRUE FISHES (Pisces). 
* General Characteristics—Aquatic Vertebrates with a 
cartilaginous skeleton, as in the shark, or a bony one, as 
in the perch; asa rule, scaled, and breathing by means 
of gills; limbs represented by fins. 
Skeleton.— At first glance, the skeleton of a bony fish 
(Fig. 193) seems to have two backbones ; the lower, how- 
ever, is the vertebra, that extends from the head to the 
tail. The upper series, f and ¢c, are median or middle fins, 
supported by interspinous bones. The backbone is com- 
posed of sometimes two hundred vertebre in bony fishes, 
