They occur in particular in relation with the orifice of the zooecium, and with that of the compensation-sac. This delicate structure is frequently guarded by an avicularium at its entrance, while avicularia are also commonly found on either side of the operculum or in other positions close to that structure.
Stylifer, the operculum is lost, animal fixed by a large proboscis which forms a pseudopallium covering the whole shell except the extremity of the spire, parasitic on all groups of Echinoderms. Entosiphon, visceral mass still coiled; shell much reduced, proboscis very long forming a pseudopallium which covers the whole body and projects beyond in the form of a siphon, foot and nervous system present, eyes, branchia and anus absent, parasite in the Holothurian Deima blakei in the Indian Ocean.
Shell semi-globular, with short spire; operculum calcareous, not spiral.
A few Euthyneura in which the shell is not much reduced retain an operculum in the adult state, e.g.
The operculum retains its op..
The calyx is triangular in section, pointed below, and the operculum is attached to it by hinge-like teeth.) Authorities.
Shell turriculated, with numerous whorls; aperture and operculum oval; eyes at summits of tentacles; siphon long.
Shell flattened, not umbilicated, generally smooth; operculum horny.
VIIgo, The genital somite or first somite of the mesosoma with the genital operculum (a fused pair of limbs).
Head with two long labial palps; shell ovoid; operculum horny, semicircular, carinated.