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cartilaginous fish are fertilized inside the female's body. Most fish lay eggs,
but there are some that are known as live bearers, for which fish fry emerge
from the female. Unlike bony fish, the cartilaginous fish have no swim
bladder; as a result they must swim continuously to maintain their vertical
positions in the water. All the sharks and rays fall into the cartilaginous
classification of fish.
e
a
i
FIG 4.66 Basic fish anatomy.
a: Brain. b: Gills. c: Heart. d: Liver.
e: Kidney. f : Stomach. g: Intestine.
h: Gonad. i: Spinal column.
b
c
f
g
h
d
Bony Fish
A typical osteichthyes has two sets of paired fins (the pectoral and pelvic fins)
in addition to the dorsal, caudal, and anal fins. Their gills are protected by a
bony structure located below and behind the eye sockets and known as the
gill cover or operculum . These fish possess swim a bladder that enables them
to maintain buoyancy by altering the amount of gas they hold in this chamber
and thereby maintain their depth with the water column.
Cartilaginous Fish
Unlike bony fish, chondrichthyes have no rigid skeletal structure that includes
spines radiating from the spinal column. Fish with no swim bladders are
forced to keep swimming to maintain their depth within the water. Failure
to continue swimming results in the fish sinking. Sharks' elongated and rigid
pectoral fins act as wings that create lift as their tails propel them forward.
Rays have mouths located on their undersides. Their fins are adapted into
wings that they use in a manner very similar to birds. The undulating action
creates both lift and forward thrust on the upstroke as well as the downstroke.
The dorsal fin has become a single thin extended spine that seems to serve
little purpose in either propulsion or stability.
Many modern fish have retained the basic streamlined kind of shape of their
early ancestors built around the axial notochord, though this has still resulted
in a great deal of variation in body shape: globular, elongated, some are
compressed dorsoventrally (rays), some (such as the ocean sunfish Mola mola )
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