The fish of the class Agnatha were the first in the vertebrate group to differentiate. They are currently considered primitive fish compared to sharks, rays, and bony fish; however, in relation to Protocordates, agnatha present many evolutionary novelties. Despite the word “primitive” suggesting extinction, some agnatha fish still live today. They are: the Witchfish (witches) and the Lampreys.
The lampreys can measure between 15cm and 1 meter in length, have an elongated body (similar to a snake), structured by cartilaginous tissue, and have an anal fin. When dealing with agnatha, the main references are the absence of jaws, paired fins, and no bony armor. Lampreys are ectoparasitic and detritivorous beings, meaning they feed on dead animals or organic remains, but there are also many 
records of lampreys parasitizing sharks, dolphins, and fish. The structure of the lamprey’s mouth allows them to suck and attach to food; their mouth is circular with sharp, keratinized teeth and a rasping tongue, shaped like a suction cup. They have 7 pairs of gill openings, and while parasitizing other fish, they keep their mouth fixed and attached to these fish, using these gills to perform gas exchange.
Both witches and lampreys are mainly found in saltwater environments, but during the breeding period, they swim upstream and go to freshwater environments. While witches are exclusive to salty and deep waters, they spend most of their lives buried on the ocean floor.
Witches are similar to lampreys in some aspects, including structures and body morphology. However, witches have a more flexible body, allowing them to tie themselves in knots. This ability, along with the capacity to expel mucus, helps them escape from predators. Like lampreys, witches are also detritivorous. Another similar characteristic in both lampreys and witches is the excretion mechanism, which is done through pronephros kidneys.
Adult lampreys die after spawning, and some do not even reach adulthood, dying without ever coming into contact with seawater. Fertilization occurs externally; females lay eggs on the riverbed, and males fertilize them. After the eggs hatch, lamprey larvae can remain in freshwater environments for up to 5 years, feeding and waiting for the moment of metamorphosis to head to the sea. Regarding the reproduction of witches, it is known that they are hermaphroditic. Due to the limited studies on the species, the information is still restricted.
Agnatha are the greatest example that not all vertebrates have vertebrae.
Source: www.infoescola.com, by Guilherme Kalid