Page 15 - Fish and Amphibians (Britannica Illustrated Science Library)

10 GENERAL CHARACTERISTICS FISH AND AMPHIBIANS 11

Distinguishing Features Near- Jawless Fish

fossils Of the ancient agnathans, considered
imilar characteristics define nearly all fish, with a few rare exceptions. These aquatic the first living vertebrates, only
Choanichthyes lampreys and hagfish are left.
animals are designed to live underwater, and they have a jawbone and lidless eyes and are (Sarcopterygii) are archaic
bony fish with fleshy fins. SEA LAMPREY
S cold-blooded. They breathe through gills and are vertebrates—that is, they have a spinal Some of them were the first
animals with lungs. Only a Lampetra sp.
column. They live in the oceans, from the poles to the equator, as well as in bodies of few species survive. Its round, toothed mouth
fresh water and in streams. Some fish migrate, but very few can pass from salt allows it to suck the blood of
fish of various species. There
COELACANTH
water to fresh water or vice versa. Their fins enable them to swim and move are also freshwater lampreys.
Latimeria chalumnae
in different directions. Animals such as dolphins, seals, and whales are This species was thought
at times mistaken for fish, but they are actually mammals. to have gone extinct
millions of years ago,
until one was discovered
ANTERIOR alive off the coast of South Just Cartilage
DORSAL FIN Africa in 1938; more of
This fin has stiff these fish were found later. Cartilaginous fish, such as rays and
HEAD PECTORAL FIN rays and has a sharks, have extremely flexible
One of the three Symmetrical, relatively small, stabilizing skeletons with little or no bone.
EYES main divisions of and with a radial structure function. SCALES
On the side of the its body The scales are POSTERIOR
NASAL PITS head, protected by imbricateΩthat DORSAL FIN
Also called fatty membranes is, they overlap This soft-structured RAY
nares; lie on one another. fin is located
either side of between the dorsal Raja miraletus
the head fin and the tail. Its large fins send
currents of water
carrying plankton and
small fish to its mouth.
The ray is very fast.

LATERAL LINE
Fish have sensory organs
all along this line.
With Spines
MOUTH
The angle of the mouth Osteichthyes is the most numerous
affects what the fish class of fish. The skeleton has some
can eat. level of calcification.
OPERCULUM
A bony flap that covers GILLS ATLANTIC MACKEREL
the gills and helps The fish's Scomber scombrus
regulate water flow breathing organs PELVIC FINS
These permit the fish This fish has no teeth. It lives in
to swim upward and temperate waters, and its meat is
downward. considered delicious. It can live
ANAL FIN
Soft, with a row for more than 10 years.
of finlets
TAIL MUSCLE
This is the
Gill Breathing strongest muscle
in the fish.
CAUDAL FIN
Gills are the organs that fish It moves from side
Filaments Water flow to side, propelling
use to breathe. They are made Gill raker Oxygenated
of filaments linked by the gill arches. blood the fish forward.
The fish uses its gills to take in
oxygen dissolved in the water.
Through a process known as diffusion, In Action Water Open Water Closed
mouth
mouth
oxygen is transferred to the blood, Capillary Water enters the mouth and
which has a lower concentration of Opening at tubes flows over the gills. After the gills Pharynx
edge of the
oxygen than the water. In this way extract oxygen, the water is
operculum Blood expelled through the gill slits. 25,000
the fish oxygenates its blood, flow
which then circulates to the rest of
Gills
its body. In most bony fish Is the number of known fish
(osteichthyes) water flows in through Gill arch Operculum Esophagus Closed Open species, making up nearly one
the mouth, splits into two streams, Gill filament Deoxygenated Opens and closes the operculum operculum half of all chordate species.
and exits through the gill slits. blood Filaments openings where water exits