Fish

Jawless Fishes

Jawless fishes evolved before jawed fishes. Among the earliest jawless fishes (Agnatha) were the ostracoderms. They were covered with hardened external plates but did not have a well-developed endoskeleton. The lived on the ocean bottom where they were filter feeders.

Descendants of the early jawless fish are present today in the lampreys and hagfishes. Both have a cylindrical, eel-like body with no paired fins. A notochord and cartilaginous skeleton are present. Hagfishes are scavengers that look like large worms with "feelers" around the mouth. Lampreys are parasitic on other fish, attaching to them with an oral, suckerlike disk.

REVIEW: The first vertebrates included ____ .
a. bony fishes
b. jawless fishes
c. lobe-finned fishes
d. both a and b
Answer b