Zooid structure

Zooid internal structure (sagittal plane). Adapted from J.S. Ryland, 1970.

Each autozooid consists of a cystid and a polypide. The cystid includes the hard calcareous shell plus the living body wall--muscles, mesothelium, basement membrane, and epidermis--while the polypide is the rest of the body, including the lophophore, introvert, gut, gonad, and funiculus.  The polypide can be recycled multiple times, each time regenerating from the remaining cystid.  Studies of Antarctic cheilostomes show that as zooids age, the life time of a polypide decreases (Barnes & Clarke 1998).  The calcium carbonate skeleton of each zooid is secreted by the epidermis (Ruppert et al. 2004).

When observing a colony, the lophophores are the bells of tentacles pulsing independently of each other, giving the branches a fuzzy appearance.  Autozooids are the only individuals in the colony that posses a lophophore, which sits atop an eversible introvert.  Based only on a casual count using photos of R. graeffei from Heron Island, the lophophores tend to comprise of 10 to 12 tentacles, each with two rows of short, frontal and long, lateral cilia.  The lateral cilia beat downwards and outwards, creating water currents that sweep particles towards the mouth and between the bases of the tentacles, whichare rectangular in cross-section.  When feeding, the introvert can be propelled from the coelomic cavity using hydrostatic pressure, pushing open the operculum (a hinged fold of the frontal membrane). Within the Ascophora, the hydrostatic pressure is created using a small sac (ascus) that lies under the calcified frontal shield.  This sac fills with water, pushing the lophophore outwards.  Logically enough, retractor muscles retract the introvert and lophophore back into the body cavity, while occlusor muscles close the operculum, sealing the polypide within the protective carbonate enclosure when not feeding (Ryland 1970).