Hagfish and Lampreys
Hagfish
Figure 2. Pacific hagfish
"Pacific hagfish Myxine" by U.S. National Oceanic and Atmospheric Administration, Wikimedia Commons is in the Public Domain
Hagfish are marine craniates of the class Agnatha or Myxini, also known as Hyperotreti. Some researchers regard Myxini as not belonging to the subphylum Vertebrata. That is, they are the only living animals that have a skull but not a vertebral column.
- Despite their name, there is some debate about whether they are strictly fish, since they belong to a much more primitive lineage than any other group that is placed in the category of fish (Chondrichthyes and Osteichthyes).
- Hagfish have a cartilaginous skull but no jaw or vertebrae. A strong, flexible notochord and segmented muscles create snake-like swimming movements in adults. They also have a small brain, eyes, ears and a nasal opening connected to the pharynx.
- They feed as a bottom dwelling scavenger. When threatened, they can release several liters of slime from glands on their flanks. The slime absorbs water and can cover the gills of fish and cause suffocation. Hagfish may also use their slime glands to repel other scavenger species while feeding.
Lampreys
Figure 3. Lampreys
"Diversas lampreas.1 - Aquarium Finisterrae" by Drow_male, Wikimedia Commons is licensed under CC BY-SA 3.0
The lamprey ingests the blood of fish by boring holes through tissues with its mouth and tongue.
- Lampreys and hagfishes form a monophyletic group representing the earliest extant primitive vertebrates. Today, they are found in both marine and freshwater environments.
- Most lampreys are parasitic organisms, with a round jawless mouth and a rasping tongue. They feed by clamping their mouth on a fish and penetrating the skin with their tongues to ingest blood.
- Lampreys inhabit both freshwater and marine habitats.
- Most species will migrate to seas or lakes.
- Lamprey skeletons are made of cartilage that is composed of a stiff protein matrix, rather than collagen, as in most vertebrates. The notochord is present in adults and has a flexible sheath surrounding it.
- Projections of cartilage, related to vertebrae, protrude along this sheath dorsally and partially enclose the nerve cord.