NERVOUS SYSTEM 
75 
through the gill opening. The mouth is thus emptied of 
water so that when it is opened again more water flows in. 
60. Circulation. — The blood of fishes is carried in well- 
defined blood vessels and a heart of two chambers. The 
blood is sent from the heart to the gills, where it is purified 
of carbon dioxide and receives oxygen. It is then carried 
by means of arteries to other parts of the body, where the 
oxygen in turn is given up and carbon dioxide is received. 
The blood from the gills and other parts of the body is re¬ 
turned to the heart through veins. Because the blood of 
fishes is at a lower temperature than the blood of man and 
changes with the seasons, they are called cold-blooded animals. 
61. Excretion. — The waste produced in all parts of the 
body as a result of the use of oxygen in the process of 
oxidation is in the form of a gas known as carbon dioxide. 
This gas is carried in the blood to the gills where it passes 
by osmosis through the thin walls of the gill filaments into 
the water. In addition to oxidation, there are other vital 
processes taking place in all the cells of the body of the fish. 
These vital processes produce waste substances that exist 
in the form of liquids which are gathered up by the small 
blood vessels and carried to the kidneys of the fish. Here 
these liquid wastes are extracted from the blood and are 
cast off from the body. 
62. Nervous System. — The nervous system of a fish con¬ 
sists of a spinal cord and a well developed brain. There is 
no structure in the nervous system of a crayfish that can 
be compared to the brain of a fish. Many nerves connect 
the brain and spinal cord with all parts of the body and 
these nerves belong to the nervous system. Associated 
with a better development of the brain are special sense 
organs. 
Special Senses. — The eye is well developed. It is glob¬ 
ular and projecting, and it is believed to be near-sighted. 
The organs of smell are usually located in the nasal cavity. 
