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How do bryozoans pull their tentacles inside their bodies?

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Eurystomella bilabiata, a moss animal from the Order Cheilostomata. — moss animal *Eurystomella bilabiata*, a moss animal from the Order Cheilostomata.

When bryozoans are disturbed or not feeding, they withdraw their tentacles—collectively known as the lophophore—into the body cavity.12

This retraction process involves the inrolling of the tentacle sheath (also called the introvert) as the mouth and tentacles are pulled downward by the action of paired retractor muscles.1

The mechanism for extending these structures back out varies by group:
* Phylactolaemates use a muscular and contractile body wall to increase hydrostatic pressure.1
* Gymnolaemates utilize flexible body walls or a specialized membranous cavity called an ascus, which can be inflated with seawater to compress the body fluid and force the tentacles outward.1
* Stenolaemates possess rigid, calcareous tubes that cannot be inflected; instead, they rely on muscles to force body fluid from one part of the zooid to another to achieve eversion.1