The visceral mass was accordingly termed the "polypide" and the body-wall which contains it the "zooecium."
This view depended principally on the fact that the life of the polypide and of the zooecium are not coextensive.
On the degeneration of the polypide, its nutritive material is apparently absorbed for the benefit of the zooid, while the pig mented substances assume a spheroidal form, which either remains as an inert "brown body" in the body-cavity or is discharged to the exterior by the alimentary canal of the new polypide.
This is formed as a two-layered "polypide-bud," which usually develops from the inner side of the zooecial wall, and soon occupies the place of the previous polypide.
The inner layer of the polypide-bud gives rise to the structures usually regarded as ectodermic and endodermic, the outer layer to the mesodermic organs.
The polypide consists of a "lophophore" bearing a series of ciliated tentacles by which Diatoms and other microscopic bodies are collected as food, of a U-shaped alimentary canal, and of a central nervous system.
In other cases the reproductive cells perhaps pass out by the atrophy of the polypide, whereby the body-cavity may become continuous with the exterior.
7, ect), which later invaginates to form the inner vesicle of the polypide-bud.
The production of a polypide by the statoblast thus differs in no essential respect from the formation of a polypide in an ordinary zooecium.
The evolution of the arrangements for protruding the polypide seems to have proceeded along several distinct lines: (i.) In certain species of Membranipora the "frontal membrane," or membranous free-wall, is protected by a series of calcareous spines, which start from its periphery and arch inwards.
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.
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.
The polypide is formed, as in an ordinary zooecium after the loss of its polypide, from a polypide-bud.