Membraniporella, Cribrilina, Cellaria, Micropora, Selenaria, Um bonula (fig.
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.
In Cribrilina similar spines C FIG.
A, Membranipora (after Nitsche); B, Cribrilina; C, Some of the Lepralioid forms. b.c., Body-cavity.
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.
(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).
The arrangement is perhaps derivable from a Cribrilina-like condition in which the outer layer of the spines has become membranous while the spines themselves are laterally united from the first.