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CLASS IV. PISCES. 
Remarks. In this class the process answering 
to respiration is performed by presenting the blood 
to the air contained in water by means of the gills. 
In effect the gills are as lungs exposed inside out. 
Fish cannot live in water, from which air has been 
expelled by boiling, until it has been exposed a 
considerable time tu the atmosphere, or been agi- 
tated init. ‘They take water into the mouth and 
compress it upon the gills by the gill-covers ; 
whereby they bring the air contained in it into 
close contact with the gills. 
Most species of fish have an air-vessel which 
they can compress and dilate at. pleasure; by 
which operation they ascend and descend in water 
without much exertion. Vhey move in water by 
their fins, w hich they use like oars, or wings. 
"Those fins\gghich answer to fore feet are called 
pectoral fint-to bind feet, caudal fins—those on 
ihe back, dorsal fins—those near the vent, anal’ 
fins—those of the abdomen, ventral fins. The ribs 
‘in the fins.and gills are called rays. These rays 
_ are thorny, soft, articulated, or branched. 
Some fish are migratory like birds ; as the shad. 
They move in vast numbers from the ocean into 
fresh water rivers in the spring season, and depos- 
ite their eggs. Some reside permanently in fresh 
water, some in salt water, and others seem to live 
in either indifferently. Those which pass into. 
fresh water rivers in the spring season, encrease 
in number in proportion to the extent‘of agricul- 
tural improvement in the countries through which 
the rivers run. The shad of the Hudson have en- 

