The ovicells commonly found as globular swellings surmounting the orifices are not direct modifications of zooecia, and each typically contains a single egg or embryo.
- Zooecia usually short.
- Zooecia prismatic or cylindrical, with terminal, typically circular orifice, not protected by any special organ.
The ovicells are modified zooecia, and contain numerous embryos which in the cases so far investigated arise by fission of a primary embryo developed from an egg.
- Zooecia with soft uncalci fled' walls, the external part of the introvert being closed during retraction by a membranous collar.
Zooecia either arising from a stolon, without lateral connexion with one another, or laterally united to form sheets.
g, zooecia; x, imperfectly developed ovicell.
- Zooecia with more or less calcified walls.
- Part of a branch of Bowerbankia pustulosa, showing the thread-like stolon from which arise young and mature zooecia.
- Zooecia of Umbonula pavonella, showing a pair of minute avicularia on either side of the orifice of each zooecium.
The alternate increase and diminution of volume is easily understood in forms with flexible zooecia.
- Diagrammatic Longitudinal Sections of Cheilostomatous Zooecia.
As indications that the conditions described in Membranipora and Cribrilina are of special significance may be noted the fact that the ancestrula of many genera which have well-developed compensation-sacs in the rest of their zooecia is a Membranipora-like individual with a series of marginal calcareous spines, and the further fact that a considerable proportion of the Cretaceous Cheilostomes belong either to the Membraniporidae or to the Cribrilinidae.
In the majority of Cheilostomes, the avicularia are, so to speak, forced out of the ordinary series of zooecia, with which they are rigidly connected.
But avicularia or vibracula may also occur in other places - on the backs of unilaminar erect forms, along the sutural lines of the zooecia and on their frontal surface.
In the Cyclostomata, further distinguished by the cylindrical or prismatic form of their highly calcified zooecia, the orifice is typically circular, without any definite closing organ.
- Zooecia, long and coherent, prismatic or cylindrical, with terminal orifices, their wall thin and simple in structure proximally, thickened and complicated distally.
Orifices of the zooecia often separated by pores (mesopores).
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