Cyclostomata
1. (subphylum Ectoprocta, class Gymnolaemata) An order of bryozoans in which the colony is made up of thin-walled, finely perforate, calcareous tubes. The aperture of each individual is rounded, without any covering. Reproduction and larval development take place in a single, enlarged individual called an ‘ovicell’. The order appeared in the Ordovician and exists at the present day.

2. The group that comprises the jawless fish, the most primitive of living vertebrates, characterized by the absence of jaws and paired fins, by a series of gill pouches rather than gill slits, by a cartilaginous skeleton without proper vertebrae, and by never having more than two semicircular canals. The taxonomic status of the term ‘Cyclostomata’ is somewhat confused: at various times it has been ranked as an order, superorder, subclass, and class. The extant orders usually included are the lampreys (Petromyzoniformes) and the hagfish (Myxiniformes) but there is now considerable doubt whether they are closely related to each other; the hagfish may be a sister group to other vertebrates.

Cyclostomata See Agnatha.