The Ctenostomata are an order of bryozoans in the class Gymnolaemata

Gymnolaemata

Gymnolaemata is a class of the phylum Bryozoa. Gymnolaemata typically live under seawater and grow on surfaces of rocks, kelps, and even in some cases, on animal species like fish. They are mostly marine bryozoans with cylindrical or flattened zooids. The lophophore is protruded by action of...

. The great majority of ctenostome species are marine, although Paludicella inhabits freshwater.

They are distinguished from their close relatives, the cheilostomes

Cheilostomata

Cheilostomata, an order of Bryozoa in the class Gymnolaemata, are exclusively marine, colonial invertebrate animals. Cheilostome colonies are composed of calcium carbonate and grow on a variety of surfaces, including rocks, shells, seagrass and kelps. The colony shapes range from simple encrusting...

, by their lack of a calcified exoskeleton. Instead, the exoskeleton is chitin

Chitin

Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

ous, gelatinous, or composed only of a soft membrane, and always lacks an operculum

Operculum (bryozoa)

In the bryozoan order Cheilostomata, the operculum is a calcareous or chitinous lid-like structure that protects the opening through which the polypide protrudes....

. Colonies of ctenostomes are often composed of elongated, branch-like stolon

Stolon

In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...

s, although more compact forms also exist.