The agnatha are also known as the jaw less vertebrates. They do not posses jaw and are primitive type of organisms. They have notochord throughout their life. They are cold blooded and have no appendages and exoskeleton. The small neural arches which extend from the notochord present the vertebral column. They have only one nostril and a couple of semi circular canals in the ear. They have a single class known as cyclostomata.
The cyclostomata are also known as the circular mouth organisms. They are mainly marine and do not have a jaw. They have ill formed head. Their body is soft and resembles an eel. They do not have scales but have a compressed tail. The fins are unpaired. They have no stomach. The endoskeleton is formed by the cartilage. The process of excretion and respiration occurs by a pair of kidneys which are mesonephric and gills respectively. They have multiple cranial nerves. Their heart is double chambered. The development is mainly indirect with the formation of larva. They have an external mode of fertilization. They act as a parasite on fishes. They suck the blood of fish by attaching them to fishes.
