Fish are water animals that evolved about 500 million years
ago. They were the first animals to have an internal skeleton. Most fish have
scale-covered bodies with fins and a tail for swimming. They breathe using
gills to absorb oxygen from the water, although a few, such as the lungfish,
can survive in air. The four classes of fish – jawless fish, sharks,
lungfish, and bony fish – have common characteristics, but are only
distantly related.
The manta ray looks strange, but is a harmless filter feeder. Large
flaps on each side of its head channel water into its mouth, and the gills
filter out animal plankton and small fish for the ray to swallow. It moves
through the water by beating its powerful, triangular pectoral fins. It can
accelerate suddenly and leap out of the water, if threatened.
The African lungfish does not have gills. It lives in stagnant water
and breathes air with its lungs. The African lungfish survives droughts by
burrowing into mud and making a cocoon where it lies dormant until the rains
fill the pool again.
The sea lamprey has a circular sucker instead of a mouth. Around the
edge of the sucker are rows of small teeth, while larger teeth surround the
opening. The lamprey sucks blood with its rough tongue. Lampreys do not have
jaws or scales, and their internal skeleton is a stiffened rod, or notochord.
The term fish is an informal grouping of chordate animals (with
backbones), living in water. They are ectothermic (cold-blooded) and move using
fins. There are about 25,000 species.
(jawless
fish)
Features: sucker-like mouth,
(cartilaginous
fish, including sharks, skates, rays)
Features: skeleton of cartilage,
tooth-like scales
(bony
fish)
Features: bony skeleton, flexible fins,
swim bladder
(lungfish)
Features: lungs, internal
nostrils
