Cystoidea is a

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

of extinct

crinozoa Crinozoa is a subphylum of mostly sessile echinoderms, of which the crinoids, or sea lilies, are the only extant members. Crinozoans have an extremely extensive fossil history, which may or may not extend into the Precambrian (provided the enigm ...

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

, termed cystoids, that lived attached to the sea floor by stalks. They existed during the Paleozoic Era, in the Middle Ordovician and

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

Periods, until their extinction in the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

Period.

Description

Cystoids are distinguished from other echinoderms by triangular pore openings. Superficially, cystoids resembled crinoids, but they had an ovoid, rather than cup-shaped, body. The mouth was at the upper pole of the body, with the opposite end attached to the substratum, often by a stalk, although some stalkless species did exist. The

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

lay on the side of the body. Five, or less commonly three, ambulacral areas ran along the outside of the body, radiating outwards from the mouth. A number of small tentacles either surrounded the mouth, or projected outwards in a row from the ambulacral areas, depending on species. The most distinctive feature of cystoids was the presence of a number of pores in the rigid skeleton encasing the body. These were most likely respiratory in nature, allowing fluid to flow in or out of the body. In some species, the pores were clustered in distinct regions, but in others they were distributed quite widely over the body surface. See also

List of echinodermata orders This List of echinoderm orders concerns the various classes and orders into which taxonomists categorize the roughly 7000 extant species as well as the extinct species of the exclusively marine phylum Echinodermata. Subphylum Crinozoa Cla ...

References

* * Paleozoic echinoderms Ordovician echinoderms Silurian echinoderms Devonian echinoderms Middle Ordovician first appearances Devonian extinctions Prehistoric echinoderms of Europe Prehistoric echinoderms of North America {{paleo-echinoderm-stub