_a_, branched spine covering front of membranous area; _b_, avicularium; _c_, vibraculum. C. Back surface; _a_, vibracular cell; _b_, vibraculum. ] In _Caberia ellisii_ the vibracular cells are very large. The vibracula, which are long and serrated, have been observed to move in unison like a double row of oars. [Sidenote: Case A 1, 2.] _Section B._ FLUSTRINA.—In this group the colonies form leafy lamellæ, crusts, etc., in which the individual cells are typically in the form of oblong boxes with their front walls wholly or partly membranous. _Flustra foliacea_ has already been described. [Sidenote: Case A 1.] In _Flustra carbasea_ the fronds are formed of only one layer of cells, and not of two layers back to back as in _F. foliacea_. The fine specimen of _Flustra nobilis_ from S. Africa is so called from the large size of its long hexagonal cells which form a honeycomb pattern clearly visible to the naked eye. [Sidenote: Case A 1.] In _Flustra cribriformis_(*) (Fig. 11), from Torres Straits, the fenestrated frond forms a beautiful spiral. _Flustra florea_, from S. Australia, grows in the form of branching tufts of narrow spiral fronds. _Electra pilosa_ [dry and spirit specimens exhibited] (Fig. 12) forms a delicate silvery lace-work, encrusting shells and seaweeds (especially red algæ) on almost every shore. The long horny spine at the base of the membranous area of each cell gives the crust a pilose appearance. In _Electra verticillata_ from West Africa, the cells form an elegant branched colony, the branches being composed of regular verticils of cells. [Illustration: Fig. 11. _Flustra cribriformis._ ] [Illustration: Fig. 12. _Electra pilosa._ A, incrusting a seaweed, natural size; B, cells magnified; _a_, lid or operculum. ] [Sidenote: 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. [Sidenote: Case A 2.]