Chordates, protochordates and fishes

All chordates have, at some stage, four features: a notochord, a dorsal hollow nerve cord (along the back), paired pharyngeal gill slits, and a post-anal tail. They are bilateral, triploblastic coelomates with closed circulation. Non-chordates are the opposite: no notochord, a solid, double nerve cord on the belly side, and a heart on the back side.

Chordates are divided by what happens to the notochord. In the marine protochordates, it stays a notochord. In Urochordata it is found only in the larva's tail (Ascidia, Salpa, Doliolum). In Cephalochordata it runs from head to tail all life (Branchiostoma, the lancelet). In Vertebrata, the notochord of the embryo is replaced by a backbone (vertebral column). So all vertebrates are chordates, but not all chordates are vertebrates.

Cyclostomata (also called Agnatha, jawless): they live as parasites on fishes, clinging with a round sucking mouth without jaws. They have 6 to 15 pairs of gill slits, and no scales or paired fins. They swim to fresh water to lay eggs, then die. Petromyzon (lamprey), Myxine (hagfish).

Chondrichthyes (cartilage fishes): marine, with a skeleton of cartilage. The mouth is on the underside. Gill slits are open, with no operculum (gill cover). Skin has tiny tooth-like placoid scales. With no air bladder, they must keep swimming or sink. Males have claspers; many give birth to live young. Scoliodon (dogfish), Pristis (sawfish), Carcharodon (great white shark), Trygon (stingray). Torpedo (electric ray) has electric organs.

Osteichthyes (bony fishes): marine and freshwater, with a bony skeleton and the mouth at the front. Four pairs of gills are covered by an operculum. Scales are cycloid or ctenoid. An air bladder controls floating. Most lay eggs. Exocoetus (flying fish), Hippocampus (sea horse), Labeo (rohu), Catla, Clarias (magur), Betta (fighting fish), Pterophyllum (angelfish). All fishes have a two-chambered heart and are cold-blooded.