912 NATURAL HISTORY. 
pose, with a considerable finny expansion, consisting of a 
skin, over a frame-work which is sometimes bony, and 
sometimes cartilaginous or gristly. It is constructed, 
therefore, very much like the wing of a bat. The fins 
are similarly constructed. These generally act chiefly as 
balancers and directors of the movement, while the scull- 
ing tail propels. That the side or pectoral fins, however, 
have considerable agency in propelling, can be seen very 
readily, if you watch the movements of fishes in an aqua- 
rium. They obviously narrow and widen as they are 
moved, widening when they make a propelling stroke. 
357. The skeletons of Fishes are not as firm as those 
of other Vertebrates. In some, even, they are not real 
bone, but are cartilaginous or gristly. The reason of this 
difference is plain. As the Fish moves in an element of 
nearly the same specific gravity with itself, it puts forth 
but little strength in its movements. The points of sup- 
port, therefore, for the muscles need not to be so firm as © 
they are in animals living in air and exerting motions 
that require considerable force, such as springing from 
the ground, grasping, flying, ete. 
358. We see a marked adaptation in the Fish to its 
mode of life in the organs of sense and the brain. Its 
life is passed mostly in obtaining its food and in escaping 
from its enemies. Its life is a lazy one compared with 
that of animals that run, and dig, and scratch, and climb, 
and fly. It shows, neither, any remarkable instincts. It 
therefore does not need much of a brain, for its range of 
thought is very limited; neither does it require acute- 
ness in the senses to meet its wants. Its brain is there- 
fore small, and the organs of sense are not as fully devel- 
oped as in some other animals. It has little sense of 
touch, and it is mostly confined to the lips. The fila- 
ments which some have about the mouth are probably 
organs of touch, informing of the contact of bodies just 
as the whiskers ofa cat do. The eyes of a fish are large 
and nearly immovable. As they are lubricated by the 
