Chondrichthyes (class) en

A class of very ancient vertebrates including sharks, rays, skates, and chimaeras, that are characterized by having a cartilaginous skeleton, and typically a notochord present in the young that is gradually replaced by cartilage, a relatively large brain weight relative to body size, red blood cells produced in the spleen and special tissue around the gonads rather than in bone marrow, unique Leydig's organ and epigonal organ that may be part of the immune system, tough skin covered with dermal teeth, five to seven gills, internal fertilization, having young produced by either live birth (ovoviviparous species) or through eggs (oviparous) depending upon the species, and by being primarily ectothermic or cold blooded. Many of the living sharks and rays are assigned to the same genera as species that swam the Cretaceous seas over 100,000,000 years ago.