Cavity of the zooecium subdivided by transverse diaphragms, most numerous in the distal portion.
This view depended principally on the fact that the life of the polypide and of the zooecium are not coextensive.
It is one of the most remarkable facts in the natural history of the Polyzoa that a single zooecium may be tenanted by several polypides, which successively degenerate.
In a few Gymnolaemata the ectocyst is merely chitinous, although in most cases the four vertical walls and the basal wall of the zooecium are calcareous.
8 A, 9 A), but in many Cheilostomes the frontal surface is protected by a calcareous shield, which grows from near the free edges of the vertical walls and commonly increases in thickness as the zooecium grows older by the activity of the "epitheca," a layer of living tissue outside it.
This tissue is continuous from zooecium to zooecium (After Allman.) FIG.
Both kinds of reproductive organs may occur in a single zooecium, and the reproductive elements pass when ripe into the body-cavity.
In Cheilostomata with a rigid frontal wall A, of Membranipora; B, of an Jullien showed that proimmature zooecium of Cribrilina trusion and retraction were p.m., Parietal muscles.
It is regarded as a modified zooecium, the polypide of which has become vestigial, although it is commonly represented by a sense-organ, bearing tactile hairs, situated on what may be termed the palate.
The operculum of the normal zooecium has become the mandible, while the occlusor muscles have become enormous.
In the vibraculum the part representing the zooecium is relatively smaller, and the mandible has become the "seta," an elongated chitinous lash which projects far beyond the zooecial portion of the structure.
In its least differentiated form the avicularium occupies the place of an ordinary zooecium ("vicarious avicularium"), from which it is distinguished by the greater development of the operculum and its muscles, while the polypide is normally not functional.
(iii.) In Umbonula of Cribrilina, showing the frontal membrane and parietal the entrance to the muscles of the young zooecium are like compensation - .sac on those of Membranipora, but they become the proximal side of the covered by the growth, from the proximal operculum (op).
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.
The polypide is formed, as in an ordinary zooecium after the loss of its polypide, from a polypide-bud.