Actinopterygii is which of the following?

• A. Ray-finned fishes
• B. Lobe-finned fishes
• C. Jawless fishes
• D. Cartilaginous fishes

Answer: (A)

Actinopterygii refers to ray-finned fishes, a major group of bony fishes. Their distinctive feature is fins that are supported by numerous slender bony rays (lepidotrichia) with a thin membrane stretched between them, giving the fins a webbed appearance. This is different from lobe-finned fishes, which have fleshy, muscular fins with a robust internal bone structure resembling limbs. Jawless fishes lack jaws entirely, and cartilaginous fishes have skeletons made of cartilage rather than bone. Because Actinopterygii are defined by their ray-supported fins within the bony-fish lineage, they are correctly described as ray-finned fishes.