SUMMARY OF ANIMAL CLASSES
CLASS MYXINI Hagfishes
Have a partial cranium (skull) but no vertebrae, and so they are not truly
vertebrates.
Their skeleton is composed of cartilage, and lacks vertebrae.
They have a notochord made of cartilage.
Hagfish also lack jaws.
They are considered vertebrates only if vertebrates are equated with the Craniata:
neural crest, paired sensory organs of the head (cephalization), brain and skull
(cranium).
There are about 30 species all of them marine, bottom-dwelling scavengers.
CLASS CEPHALASPIDOMORPHA Lampreys
There are about 35 species of lamprey living in fresh and salt water.
Lampreys are anadromous.
Have a complete cranium or braincase.
Their skeleton is entirely of cartilage.
The notochord persists in the adult and is surrounded by cartilage.
Pairs of cartilaginous projections extend upwards and partially surround the nerve
cord. These are considered
rudimentary true vertebrae.
Have a single "nostril" on the dorsal side of the
They lack paired appendages.
The larva is a suspension feeder.
Some species are parasitic on fish.
CLASS CHONDRICHTHYES Cartilaginous fishes
Probably originated in salt water.
Skin with placoid scales; their structure is similar to that of teeth.
Endoskeleton entirely cartilaginous. This is a derived characteristic.
Notochord replaced by vertebrae.
Two pairs of fins: pectoral and pelvic pairs.
Two-chambered heart.
5 to 7 pairs of gills with separate and exposed gill slits.
No swim bladder or lung.
Lateral line sensitive to changes in water pressure detects vibrations.
Internal fertilization and separate sexes.
Pelvic fins modified into claspers in males to act as an intromittent organ
Oviparous, eggs are released and the young are born outside the mother;
Ovoviviparous, fertilized eggs are kept in the uterus where they hatch;
Viviparous, you develop in the uterus and are fed by the mother through a placenta.
About 750 species: sharks, rays and chimeras
CLASS OSTEICHTHYES Bony fishes
