25.1 – Early Fishes
• The earliest fishes were jawless
fish (class Agnatha). They have eel-like bodies, cartilage
skeletons, and gill pouches.
• The first class of fish with
jaws were Acanthodians. They had an internal bony skeleton.
• The second class of fish with
jaws were Placoderms. They were covered with a heavy, bony platelike
skin.
• Class Chondrichthyes are
fishes with skeletons of cartilage. They have rough skin, replacable
teeth, jaws, and gill slits. Modern Chondrichthyes include
sharks, rays, and skates.
• Sharks and rays practice internal
fertilization.
25.2 – Bony Fishes
• Class Osteichthyes consists
of bony fishes.
• Their skeleton is mostly bone,
and they have scales.
• An operculum covers their gills
(which are used to extract oxygen from the water).
• They usually have a swim bladder
to adjust their depth in the water.
• Most of them practice external
fertilization and most lay eggs.
• There are 2 major groups of bony
fishes: lobe-finned and ray-finned.
• Lobe-finned may be closely related
to amphibians. They include 6 species of lungfish and 1 species of
coelacanth.
25.3 – Amphibians
• Amphibians were the first land
vertebrates.
• Amphibians are in 3 classes:
Anura – frogs
& toads; 3680 species; tail only in tadpole; limbs used for jumping
Urodela – salamanders
& newts; 360 species; tail in adults; carnivorous; internal fertilization
Gymnophiona
– caecilians; 160 species; worm-like body, no legs; lives only in the tropics
• Except for caecilians, all amphibians
have legs, webbed feet, and no claws.
• They use their gills, lungs,
skin, and mouth for respiration.
• They have a well-developed circulatory
system.
• Since their skin is not watertight,
they must live in moist environments. Their eggs are also not watertight.
LINK: Online
frog dissection |
