Practice question

Protective housing of individual zooids in many ectoprocts involves secretion of elaborate exoskeleton termed zooecium composed primarily of chitinous cuticle frequently impregnated with calcium carbonate forming rigid box vase or tubular case enclosing softer polypide containing lophophore and viscera. In marine Gymnolaemata calcified zooecium provides rigidity against predation abrasion, frontal wall may be membranous or calcified, while Ctenostomata maintain flexible cuticle. Some possess frontal membranes, ooecia for brooding and spines defense. Composition reflects ambient calcium availability and colony form encrusting versus erect. Protective cystid contrasts with chitinous tube of phoronids and bivalved shell of brachiopods constituting unique apomorphy.