The ascus is a diagnostic morphological feature of the

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

n suborder

Ascophora Ascophora is an infraorder under order Cheilostomatida of the Bryozoa. They are now considered a synonym of Flustrina. Ascophorans are distinguished from other cheilostomes in having a completely calcified wall covering their frontal surface ...

(hence the name of the suborder). It is a water-filled sac of frontal membrane opening (ascopore) at or near the zooid orifice. It functions as a hydrostatic system by allowing water into the space below the inflexible, calcified frontal wall (covering their whole frontal surface apart from the orifice) when the zooid everts its polypide (feeding tentacles) by muscles pulling the frontal membrane inwards (non-ascophoran cheilostomes do not need this structure as their frontal wall is not calcified and thus flexible). The ascus, along with a calcified frontal shield, define ascophoran bryozoa.

References

Cheilostomatida Protostome anatomy {{bryozoan-stub