The Cyclostomata are split into Myxinodiea and Petromyzontiformes. The Myxinodiea, or hagfish, are boneless fish that live exclusively in marine environments. Some live along the continental shelf, while others live in the deeper regions. Hagfish lack true teeth and jaws. Rather they have a keratinized teeth and tongue. This keratinized acts in a pully-like fashion, where one muscle contracts to pull it one way, then relaxes and another muscle contracts to pull it the opposite way. This causes a sliding motion which the hagfish uses to scrape away at other organisms skins and tear holes into them. Hagfish are also isosmotic, and thus have unusual kidney function as they generally do not need to do much in the way of regulation. Their eyes are small, and are largely just used for detecting light and not true eyes for visual processing. While they have no bones, they do possess a cartilage plate underneath their brain. Additionally hagfish have a lateral line system which allows them to detect the movement of water around them, or if they themselves are moving.
Petromyzontiformes, or lamprey, are similar to hagfish. Like the hagfish, lamprey’s do not have any bones and a nostril on the top of their head. However unlike the hagfish, lamprey have true eyes and one pineal eye used for light detection. Additionally they have a circular mouth which is used to suck on and attach to either prey or various objects, such as rocks. Lamprey are parasitic and survive by sucking the fluid out of other fish. As adults, lamprey live in a marine environment, though they migrate back to freshwater to spawn. Lamprey larva are known as ammocoets and live as filter feeders. These ammocoets tend to pile up into structures known as redds. Surprisingly this larval stage is the primary lifestage for lamprey. Ammocoetes live for seven to eight years before maturing into an adult lamprey. At this time the lamprey will migrate to a marine environment.
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