THE TRUE FISHES. 157 
of two chambers—an auricle and ventricle. The former re- 
ceives the impure blood and pumps it into the latter. From 
here it is sent into a chamber or arterial bulb that divides 
into four™ pairs of branches, one leading into each gill. 
The blood enters the gills, is purified, then by the branchial 
veins passes into the dorsal artery (Fig. 194, @@), so finding 
its way slowly over the body and back to the auricle. So 
slowly is this circulation accomplished that the blood rarely 
attains a temperature above that of the surrounding water.+ 
Hence the fishes are said to be cold-blooded. 
Respiration.—Lifting up the gill-cover (Fig. 194, B, gc). 
we see the gills, A, g, by which the’ bony fishes breathe. 
They are blood-red, membranous leaflets, supported by 
cartilaginous arches separated by slits, the entire arrange- 
ment placed on each side of the head in what are called 
branchial chambers. Water is taken in at the mouth, 
passes between the arches and over the gills, supplying 
oxygen to the blood that has been pumped there by the 
heart for the purpose, passing out again under the gill- 
cover (Fig. 194, B, gc), so that there is a continuous cur- 
rent of water flowing in at the mouth and out at the gills. 
Most fishes have an air-bladder (Fig. 194, A, a) that con- 
tains gases, principally nitrogen, that enable them to main- 
tain a certain specific gravity. In some fishes it has the 
attribute of a lung. 
Nervous System, etc —The nervous system of fishes con- 
sists of a small brain and spinal cord (Fig. 194, A, #72), that 
throws off delicate threads to the various parts, the fins, 
the eye, A, ¢, the ear, A, ce, and the nose-pit, 7. The na- 
sal organs, except in the lung fishes and myxinoids, do not 
connect with the mouth. They are represented by cov- 
ered pits lined with nerves. Water laden with odors flows 
* Sometimes five. 
+ In the active fishes (Scombride) the respiratory process is so ener- 
getic that the temperature of the blood is often higher than that of the 
surrounding medium. 
