Fig. 11.
Flustra cribriformis.
Fig. 12.
Electra pilosa. A, incrusting a seaweed, natural size; B, cells magnified; a, lid or operculum.
Case A 1.
Membranipora membranacea occurs in the form of horny incrustations on bladder-wrack, which, owing to their flexibility, are able to adapt themselves to the swaying of the fronds of the Fucus.
The Selenariidae (Case B 2) form free colonies, usually orbicular in shape, convex above and concave below. In Lunulites capulus alternating rows of cells and vibracula radiate from the centre of the colony.
Section C. Escharina.—In this group, the front walls of the cells are wholly calcareous. Many species form patches or crusts on shells etc., and hence the name of the section; other species, again, form stony tree-like growths, or thick plates. Frequently one and the same species occurs in the form of crusts or of erect lamellæ, the identity being recognised by the characters of the individual cells.
Often a large number of species may be found on one shell. Two good examples of this are exhibited in Case A 2.
Case A 2.
Lepralia pallasiana (Fig. 13) forms sub-circular vitreous patches on stones and shells; the cells are rather large, broadly oval, and with the front wall punctured with pores; the aperture is squarish and with a slight indentation on each side.