Name and briefly characterize the four classes of fishes. Indicate which class is considered to be most similar to the ancestral chordate and which is the ancestor of the land vertebrates, justifying your choices
What will be an ideal response?
Names and characteristics: Agnathans: jawless, skeleton of bone or cartilage, gills slits supported by skeletal arches. Placoderms: jaws, teeth, paired fins, currently extinct, gave rise to next two groups. Chondrichyes or cartilaginous fishes (sharks, skates, and other predators): cartilage replaces bone. Osteichtyes or bony fishes: most modern fish are placed in this group. We believe jawless fish resemble the ancestral chordate, in part because of their lack of pared fins and jaws, and the resemblance of their larvae to lancelets. We believe that the amphibians arose from the group of bony fish called the lobed-finned fish. These have fins supported by bones that allow them to "walk" on the lower surface of muddy ponds. They often have lungs, which allow them to spend some time out of the water.
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