Hagfishes are characterized by:
- Four pairs of tentacles surrounding the mouth and the opening for the nasopharyngeal duct.
- A duct leading from the esophagus to the exterior (oesophagocutaneous duct), on the left side only, and opening behind the rearmost gill opening.
- A ventrolateral series of very large glands which produce a slime made up by large thread cells.
There are several other characteristics by which hagfishes differ from all other vertebrates, but these are either absences (lack of extrinsic eye muscles, lack of eye lens, lack of cardiac innervation, lack of radial muscles, etc.) or conditions which recalls that in cephalochordates or tunicates (accessory venous hearts, disseminate pancreas, etc.). In both cases, these apparently unique hagfish features are likely to be primitive for all craniates.
The mouth of hagfishes is armed with a protractable and retractable cartilaginous plate which bears two pairs of comb-shaped horny teeth (the "rasping tongue"). These are used to grasp on the food and convey it toward the pharynx. The mouth is not involved in the intake of the respiratory water. Instead, the water is inhalated through the nasopharyngeal duct, which opens at the anterior tip of the head. It leads posteriorly to the median olfactory organ and then to the pharynx and gill pouches.
The skeleton of hagfishes is made up by a complex assemblage of cartilaginous bars (see figure on the Craniata page), which serve as antagonists to an equally complex head musculature. Cartilages also strengthen the tentacles, nasopharyngeal duct, velum, "tongue", but there are no branchial arches. Neither is there any braincase proper, the brain being surrounded by a fibrous sheath. Nevertheless, the olfactory organ and the labyrinth are enclosed in cartilaginous capsules. In the body, the only skeleton is the notochord and the cartilaginous rays of the caudal fin.
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