THEIR GENERAL CHARACTER AND STRUCTURE 11 
Occupying as they do the lowest place in modern classifi- 
cation of vertebrate animals possessing a skull and brain 
General (Craniata), the structure of fishes generally is of a 
Structurée more plastic nature than that of more highly 
organised creatures. This not only brings about extreme, and 
often fantastic, modification of form in compliance with 
acquired habit and physical environment, but tends in 
certain genera to render the separation of species from mere 
trivial, local, or temporary varieties exceedingly difficult and 
dubious.* 
For the central type of the class it is permissible to take 
the most highly organised of British fresh-water fish, the 
perch, although the superior swiftness and dimensions of the 
salmon, and its simpler outline, perhaps render it more satis- 
factory for that purpose. However, having borrowed Dr. 
Gunther’s excellent figure of the perch’s skeleton to illustrate 
this chapter, let us take that fish as the standard of the class. 
Here we find an animal, whereof the specific gravity is greater 
than the water, suspended without apparent effort in that 
medium in a horizontal position, but with its sides vertical, 
moving in any direction with a minimum of friction at various 
degrees of speed. The first question suggesting itself is, 
Why does not the fish, being heavier than the water, sink 
to the bottom? 
The explanation is not to be found complete in the 
external structure of the animal, but involves notice of 
Theair- 20 internal organ peculiar to some, but not to all 
bladder. fishes—namely, the air-bladder. This consists of a 
sac placed in the abdominal cavity, and is considered to be a 
rudimental form of the lungs of higher animals. This bladder, 
which in some kinds of fish is connected with the pharynx by 
a pneumatic duct, is entirely closed in the perch, at least in 
* Among British fishes this is notably the case with the Sa/monide, 
and will be more fully discussed in the chapter dealing with that 
family. 
