CHAPTER I 
FISHES: THEIR GENERAL CHARACTER 
AND STRUCTURE 
Respiration—General Structure—The Air-Bladder—Fins—An Exception in 
the Flat-Fishes—Pectoral and Ventral Fins—The Vertical Fins— 
Divisions of the Body—External Design—Locomotion and its Organs 
—Scales—The Lateral Line. 
To all mammals, birds, and to many reptiles, insects, and other 
animals, oxygen dissolved in water is useless for purposes of 
respiration ; and although there are certain warm-blooded 
vertebrate creatures, so remotely related to each other in the 
modern system of classification as the whale, the manatee, and 
the seal, which, having made the water their permanent habita- 
tion, have acquired many of the external characteristics of fish, 
yet all of these depend for existence upon periodical draughts 
of fresh air. Some reptiles and insects pass through their 
earlier metamorphoses, from egg to larva and pupa, in the 
water ; but on reaching the ultimate or perfect stage, they lose 
the peculiar apparatus which enables them to extract oxygen 
from their native medium ; they spend the rest of their lives 
breathing free air as terrestrial animals, and although they 
must visit the water to deposit therein the ova upon which 
the perpetuation of their species depends, they cannot remain 
submerged except on pain of death. 
The class of Fishes, on the other hand, consists of those 
vertebrate animals which inhabit salt or fresh water, extracting 
therefrom the oxygen necessary to their existence by means of 
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