The ascus is a diagnostic morphological

Morphology (biology)

In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 feature of the bryozoa

Bryozoa

The Bryozoa, also known as Ectoprocta or commonly as moss animals, are a phylum of aquatic invertebrate animals. Typically about long, they are filter feeders that sieve food particles out of the water using a retractable lophophore, a "crown" of tentacles lined with cilia...

n suborder Ascophora

Ascophora

Ascophora is a infraorder under order Cheilostomata of the Bryozoa. Ascophorans are distinguished from other cheilostomes in having a completely calcified wall covering their frontal surface apart from the orifice, and possessing an ascus . The ascus is a water-filled sac of frontal membrane...

 (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

Polypide

The polypide in bryozoans encompasses most of the organs and tissues of each individual zooid. This includes the tentacles, tentacle sheath, U-shaped digestive tract, musculature and nerve cells. It is housed in the zooidal skeleton, which in cyclostomes is tubular and in cheilostomes is box-shaped....

 (feeding tentacles) by muscles pulling the frontal membrane inwards (non-ascophoran 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...

do not need this structure as their frontal wall is not calcified and thus flexible).