494 
THE OCEAN WORLD. 
These fins are always nearly of the same structure, consisting gene- 
rally of a fold of the skin, supported by slender, flexible, cartilaginous 
or osseous rays, connected by a thin membrane. 
Fig. 330. Skeleton of the Common Perch. 
a, the inter-maxillary bone ; 6, 'the maxillary bone ; d, the gills ; c, the under jaw ;/, the inter-operculum ; 
g g\ the vertebral column; h, the pectoral tin; i, the ventral tin; k and l, the dorsal fins; m, the anal 
nn ; n, the caudal fin. 
The muscles, which hind together the vertebral column, are so 
much developed in fishes as well as others of the superior animals, 
that they constitute in them alone the principal part of the body. Tb e 
caudal, dorsal, and anal tins act as outlying oars ; the pectoral and 
ventral fins assist in progression, at the same time that they help 1° 
maintain the equilibrium of the animal and guide and direct its more' 
merits, which are generally astonishing from their rapidity. 
An organ, which belongs properly to fishes (Fig. 340), and which is 
usually considered as their chief aid in swimming, is a large bladder 
Fig. 310 Swimming bladder of the Carp. 
situated within the body, between the dorsal spine and the abdomen 
This is usually called the swimming bladder. According to the 
volume this bladder assumes, the animal can increase or diminish the 
specific gravity of its body; that is, it can remain in equilibria 111 
