Which of the following characteristic features always holds true for the corresponding group of animals?

Chondrichthyes is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. Viviparous - Giving birth to living young that develops within the mother's body rather than hatching from an egg. It is the characteristic feature of Mammalia. Cyclostomata belongs to the Phylum Chordata that lack jaws. All reptiles have three-chambered hearts, except crocodiles, which have four-chambered hearts, like mammals and birds.