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Cyclostomi, often referred to as Cyclostomata , from Ancient Greek κύκλος (kúklos), meaning "circle", and στόμα (stóma), meaning "mouth",
Biologists historically disagreed on whether cyclostomes are a clade. The "vertebrate hypothesis" held that lampreys are more closely related to Gnathostomata than they are to the hagfish. The "cyclostome hypothesis", on the other hand, holds that lampreys and hagfishes are more closely related, making Cyclostomi monophyletic.
Most studies based on anatomy have supported the vertebrate hypothesis,
while most molecular phylogenies have supported the cyclostome hypothesis.
There are exceptions in both cases, however. Similarities in the cartilage and muscles of the tongue apparatus also provide evidence of sister-group relationship between lampreys and hagfishes.
Several groups of Palaeozoic jawless fish have been suggested to be more closely related to cyclostomes than to jawed fish, including and Anaspida. The presence of mineralised elements in these jawless fish, like the oral conodont elements and the armoured body covering of anaspids and scutes on other species like Lasanius suggests that mineralised tissues were present in the last common ancestor of all vertebrates, but were secondarily lost in hagfish and lampreys.
Hagfishes have direct development, but lamprey go through a larval stage followed by metamorphosis into a juvenile form (or adult form in the non-parasitic species). Lamprey larvae live in freshwater and are called ammocoetes, and are the only vertebrates with an endostyle, an organ used for filter feeding that is otherwise found only in and . During metamorphosis, the lamprey endostyle develops into the thyroid gland.
The Cyclostomi evolved oxygen transport hemoglobins independently from the jawed vertebrates.
Hagfishes and lampreys lack a thymus, spleen, myelin and sympathetic chain ganglia. (1998). 9789401064651 ISBN 9789401064651 Neither species has internal eye muscles and hagfishes also lack external eye muscles. Both groups have only a single olfactory organ with a single nostril. The nasal duct ends blindly in a pouch in lampreys but opens into the pharynx in hagfishes. The branchial basket (reduced in hagfishes) is attached to the cranium.
The common ancestor of both cyclostomes and gnathostomes went through a genome duplication before their split, and while a second genome duplication occurred in the stem-gnathostomes, the stem-cyclostomes experienced an independent genome triplication.
The mouth apparatus in hagfishes and adult lampreys has some similarities, but differ from one another. Lampreys have tooth plates on the top of a tongue-like piston cartilage, and the hagfish have a fixed cartilaginous plate on the floor of its mouth with groves that allows tooth plates to slide backwards and forwards over it like a conveyor belt, and are everted as they move over the edge of the plate. Hagfishes also have a keratinous palatine tooth hanging from the roof of the mouth. Hagfish - Cronodon
Unlike jawed vertebrates, which have three semicircular canals in each inner ear, lampreys have only two and hagfishes just one. The semicircular canal of hagfishes contains both stereocilia and a second class of hair cells, apparently a derived trait, whereas lampreys and other vertebrates have stereocilia only. Because the inner ear of hagfishes has two forms of sensory ampullae, their single semicircular canal is assumed to be a result of two semicircular canals that have merged into just one.
The hagfish blood is isotonic with seawater, while lampreys appears to use the same gill-based mechanisms of osmoregulation as marine . Yet the same mechanisms are apparent in the mitochondria-rich cells in the gill epithelia of hagfishes, but never develops the ability to regulate the blood's salinity, even if they are capable of regulating the ionic concentration of Ca and Mg ions. It has been suggested that the hagfish ancestors evolved from an anadromous or freshwater species that has since adapted to saltwater over a very long time, resulting in higher electrolyte levels in its blood.
The lamprey intestine has a typhlosole that increases the inner surface like the spiral valve does in some jawed vertebrates. The spiral valve in the latter develops by twisting the whole gut, while the lamprey typhlosole is confined to the mucous membrane of the intestines. The mucous membranes of hagfishes have a primitive typhlosole in the form of permanent zigzag ridges. This trait could be a primitive one, since it is also found in some such as Ciona. The intestinal epithelia of lampreys also have ciliated cells, which have not been detected in hagfishes. Because ciliated intestines are also found in Chondrostei, lungfishes and the early stages of some teleosts, it is considered a primitive condition that has been lost in hagfishes.
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