Some members have a cartilaginous endoskeleton?

The correct answer is Chondrichthyes, which refers to a class of animals commonly known as cartilaginous fish, including sharks and rays. These organisms possess a skeleton made primarily of cartilage rather than bone. This cartilaginous structure allows for certain advantages, such as lighter body weight and greater flexibility in movement, which are beneficial for their aquatic lifestyle.

In evolutionary terms, Chondrichthyes diverged early from the lineage that led to bony fish and other vertebrates. This class exemplifies adaptations that allow these fish to thrive in various marine environments.

In contrast, amphibians and mammals have bony skeletons, which provide different structural support and advantages suited for their specific terrestrial lifestyles. Birds (aves) also have a bony skeleton, adapted for flight, with many of the bones being lightweight and hollow.