Cartilage fish make blood outside the bone
Chondrichthyes names jawed fishes whose skeletons stay mostly cartilage—sharks, rays, skates, sawfish, and chimaeras. Without bone marrow, they brew red cells in spleen, epigonal tissue, and Leydig's organ. Placoid denticles armor skin while heterocercal tails power many sharks.
The class splits into Elasmobranchii, meaning sharks, rays, skates and sawfish, which lack gill covers, and Holocephali, the chimaeras or ghost sharks. Living members span from the 10-centimetre finless sleeper ray to the whale shark at over 10 metres. In most lineages the notochord gives way to vertebrae during development, but chimaeras keep theirs intact, and some deepwater sharks have a reduced spine. Chimaeras also lack the Leydig's and epigonal organs that make blood in other members.
Except for soft-skinned electric rays, the skin is armoured with placoid scales, tiny tooth-like denticles usually aligned one way, so it feels smooth stroked tail-ward and like sandpaper the other way. Whether mouth teeth evolved from migrating skin denticles or the reverse is unresolved, because the older placoderms had bony cutting plates instead of teeth. Five to seven pairs of gills handle breathing: many open-ocean species must keep swimming to ventilate, while bottom dwellers pump water through spiracles behind the eyes.
Senses are formidable. Jelly-filled pores called ampullae of Lorenzini detect electric fields, a lateral line picks up vibration and pressure, and most species have large eyes and powerful noses. Fertilisation is internal; most bear live young, some lay eggs, and none care for offspring after birth. Fishing can trigger premature birth or abortion in at least 12 percent of live-bearing sharks and rays, a toll fisheries rarely account for.
The oldest unambiguous jawed-vertebrate fossils, Qianodus and Fanjingshania from China about 439 million years ago, belong to this lineage, which descends from spiny acanthodian ancestors. Palaeozoic seas held far more chondrichthyan diversity than today's.
Source: Chondrichthyes
