1. with a dorsal hollow nerve chord.
2. with a notocord at some time in their life history.
3. with pharyngeal gill slits at some time in their life history.
example; tunicates or sea squirts
1. Adults sessile-larvae motile.
2. Notocord present only in the larvae and restricted to the posterior.
3. Filter feeders.
Example; amphioxus or lancelets.
1. Notocord extends form the head to the tail ( anterior to posterior ) and is present in both the larvae and the adult.
2. Motile filter feeders.
1. With a serial arrangement of vertebrae.
Class Agnatha (jawless ) or Cyclostomata ( round mouth )
Examples; lamprey, hagfish
1. No true jaws-round mouth.
2. No true teeth-horny teeth composed of protein and located in the roof of the mouth. They are used to rasp the scales and skin from the surface of a fish exposing blood vessel so that they can suck the blood.
3. Adults parasitic feeding on blood-larvae filter feeders.
4. No scales -leathery skin.
5. No paired fins-single mid-dorsal fin.
6. With a protocercal tail.
7. No swim bladder.
8. Sensory pits restricted to the face.
9. With 7 pair of unprotected gill slits.
10. Notocord present throughout their life history.
12. External fertilization.
13. With extended larval stages.
Class Chondrichthyes ( cartilaginous fishes), examples: shark, skates and rays
1. With true jaws which scientists believe evolved from the first pair of gill bars.
2. With denticles or placoid scales.
3. With true teeth which are composed of calcium and which are grown in rows throughout the life of the shark and which are periodically shed. Scientists believe that the teeth evolved from the placoid scales or denticles.
4. With paired fins-pectoral fins and anal or pelvic fins.
5. Caudal fin heterocercal.
6. No swim bladder.
7. Sensory pits along the side forming the lateral line.
8. With Ampullae of Lorenzinii which are electroreceptors that detect low level electrical fields produced by the prey.
9. With 5 pair of unprotected gill slits.
10. Notocord replaced in the embryo by vertebrae composed of cartilage.
11. Dioecios and sexually dimorphic.
12. With internal fertilization involving copulation.
13. Oviparous or oviviparous.
Class Osteichthyes ( bony fishes )
Examples: flounder, perch, grouper, redfish, tuna etc.
1. With true jaws.
2. With true teeth composed of calcium.
3. With paired fins.
4. With ctenoid, cycloid, or ganoid scales.
5. With one pair of gill slits protected by the operculum.
6. With a homocercal Caudal fin.
7. With a swim bladder which provides buoyancy.
8. Notocord replaced in the embryo by vertebrae composed of bone.
9. Dioecious or protandric.
10. 10. With internal or external fertilization.
11. Oviparous or oviviparous.