156 ANIMAL FORMS 
bass and perch and many other fishes as a float to enable 
them, without much effort, to remain suspended at a defi- 
nite level. By compressing this sac, partly by its own mus- 
cles and partly by those of the body-wall, the bulk of the 
fish is made less, and it sinks; upon the relaxation of these 
same muscles the body expands and rises again. Deep-sea 
fishes, when brought to the surface, where the pressure is 
relatively slight, are found with their air-bladders so dis- 
tended that they can not sink again, and the float of surface 
fishes would be as useless if they were to be carried into the 
depths below, so that such fishes are compelled to keep 
within tolerably definite limits of depth. Morphologically 
considered, the air-bladder is a modified or degenerate lung, 
and in many fishes it is lost altogether. 
148. Respiration — Looking down the throat of the perch, 
or any other fish, a series of slits (the gill-openings), usually 
four or five in number, may be seen on each side communi- 
cating with the exterior. In the sharks these outer open- 
ings are readily seen, but in the bony fishes they open into 
a chamber on each side of the head, covered by a bony plate 
or gill-cover that is open behind. On raising these flaps 
the gills may be seen composed of great numbers of bright- 
red filaments attached to the bars between each slit. Dur- 
ing life the fish may be seen to open its mouth at regular 
intervals, and, after gulping in a quantity of water, to close 
it again, contracting the sides of the throat to force it out 
of the gill-openings and over the gill-filaments to the exte- 
rior. During this process the blood traversing the excess- 
ively thin filaments extracts the oxygen from the water and 
carries it to other parts of the body. 
With this information, let us return to the study of the 
three classes of fishes. 
149. The lancelet (Branchiostoma).—The lancelet, some- 
times called amphioxus (Fig. 98), the type of the class Lepto- 
cardit, is a little creature, half an inch to four inches long, in 
the different species, transparent and colorless, living in the 
