Even though hagfish have no vertebrae and lampreys have only rudimentary vertebrae, both have a cranium and other structures that place them in subphylum vertebrata of phylum chordata. There are about 65 species of hagfish and 41 species of lamprey currently known. Agnathans are some of the oldest, most primitive fishes, having evolved in the Ordovician period, 450-490 million years ago.
All agnathans lack paired fins and scales and have a slender, eel-like body form, but they are not eels. They are distinguished from gnathostomes, the "jawed-mouth" fish, by their lack of jaws. Their skeletons are made of cartilage and their gills are pore-like structures, unlike the gill slits found in other fish. However, lampreys and hagfish have many morphological differences, which is why they have been divided into two separate classes.
Class Myxini, the hagfish, have naked skin with slime glands. They lack a dorsal fin, have a biting mouth with two rows of eversible teeth on their tongue and 16 pairs of pore-like gills. Accessory hearts located behind the gills pump blood more efficiently. Their kidney is mesonephric; it's derived from intermediate mesoderm in the embryo. The body fluid is isosmotic; it has the same osmotic pressure as seawater. They have no cerebellum, no stomach and one pair of semicircular canals in the ear. Their eyes are degenerate, so they are nearly blind. Sexes are separate and there is no larval stage.
Hagfish are marine animals that feed on annelids, crustaceans, mollusks, and dead or dying fish. They have keen senses of touch and smell, which aid in predation and scavenging. They grasp onto prey using a toothed tongue that folds together like a pincer and rasps the animal while remaining anchored to its side. If disturbed, they secrete large amounts of milky slime, which makes them difficult to handle.
Class Cephalaspidomorphi, the lampreys, have one or two dorsal fins, a sucker-like oral disc and toothed tongue, no accessory hearts, seven pairs of pore-like gills and a small cerebellum. The kidney is opisthonephric; it develops from the main mass in the embryo. Like most fishes, lampreys regulate the osmolarity of their body fluids. They have well developed eyes and two pairs of semicircular canals in the ear.
Lampreys can grow up to 3 feet long. There are parasitic species that attach to prey and nonparasitic species. Some lampreys live in freshwater, and others are anadromous; they are marine but ascend into freshwater to breed. There, the males build nests and wait for the females. During breeding, the females attach to a rock and lays eggs into the nest, where the male fertilizes the eggs. Adults die soon after spawning. The hatchling larvae are called amnocoetes. These larvae drift downstream and are suspension feeders for three to seven years, then quickly metamorphose into adults. Marine species return to the sea as adults. Parasitic species produce an anticoagulant to suck the blood out of prey while attached. Nonparasitic species do not eat; instead they return upstream to spawn and die, completing the life cycle.