91Ó°ÊÓ

Chondrichthyes are jawed fishes that possess paired fins and a skeleton made of cartilage. This class of fish arose approximately 370 million years ago in the early or middle Devonian. Examples: Sharks, Rays and Sawfish.

Chondrichthyes are thought be descendants of Placoderms, which possessed an endoskeleton made of bones. Hence, option (b) is the correct answer.

a. a cartilaginous skeleton

Chondrichthyes possess a cartilaginous skeleton, but fishes that they descended from had bony skeleton. Hence, option (a) is the incorrect answer.

c. mucus glands

Mucus glands are present mostly in Osteichthyes, it releases mucus that reduces drag during swimming and helps in osmoregulation. Hence, option (c) is the incorrect answer.

d. slime glands

Unique slime gland are found in hagfishes, which is present beneath the skin, that releases slippery mucus and helps hagfishes to escape from their predators. Hence, option (d) is the incorrect answer.