Fig. 4.

Schematic longitudinal section of a single zooid of Cinctipora elegans with the skeletal parts of the cystid forming a tube around the polypide. The calcified parts of the skeleton are displayed in light brown. The orifice leads into the vestibulum that continues via the atrial sphincter into the atrium. The latter is formed by the inverted tentacle sheath around the retracted lophophore (dark blue). At the lophophoral base lies the mouth opening. The gut (green) is u-shaped with a short foregut leading into a voluminous caecum and further via a pylorus and intestine into the anus that opens into the tentacle sheath. Cyclostomes have a membranous sac which constitutes the peritoneum detached from the epidermis to form an exosaccal cavity (primary body cavity) and endosaccal cavity (coelom or secondary body cavity, light blue). Prominent retractor muscles origin from the body wall and insert at the lophophoral base. A muscular funiculus connects the caecum to the proximal area of the membranous sac. Abbreviations: a – anus, al – attachment ligament, ao – attachment organ, as – atrial sphincter, at – atrium, cae – caecum, cs – calcified skeleton, en – endosaccal space (coelom), ex – exosaccal space, f – funiculus, int – intestine, l – lophophore, ms – membranous sac, py – pylorus, rm – retractor muscle, v – vestibulum