Cyclostomi

This article is about the class of jawless fish. For the ancient order of stenolaemate bryozoans, see Cyclostomatida.

Cyclostomata is a polyphyletic group that describes the living jawless fish, which includes the lampreys and hagfishes.

Jawless fish do not have a jaw, of course, but they have rows of teeth that move in a circular motion; hence the name cyclostomata. Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.

Cyclostome hypothesis vs. Vertebrate hypothesis

There is much conflict on the relationship between the jawless fishes. There is some molecular evidence for the group to be monophyletic so that hagfishes and the lampreys are more closely related, the "cyclostome hypothesis". This is against the "vertebrate hypothesis" which theorizes that lampreys are more closely related to the other gnathostomes than they are to the hagfish. More data will be needed to be more confident.

References

• Related text and image resources
• Nelson, Joseph S. (2006). Fishes of the World. John Wiley & Sons, Inc. ISBN 0471250317