FLUSTRA. ESCHARA.
Flustra (Sea-mat). [Lign. 89, fig. 4, 5.]—The polyparium is either membranaceous and flexible, calcareous and encrusting, or foliaceous, composed of cells, arranged in juxtaposition, more or less quadrangular, flat, with a distinct border, disposed on a flat surface, or on opposite surfaces, as in the F. foliacea.
This is one of the most common genera of the encrusting and frondescent zoophytes. The Flustra consists of a cluster, or aggregation of polypes, invisible to the naked eye; under the microscope, the polype is found to be a transparent gelatinous body bent on itself, with a sac or digestive cavity, having two apertures, the external margin of which terminates in eight or ten tentacula, clothed with cilia; the whole is surrounded by a firm wall, constituting a cell, from which the animal can protrude its tentacula and upper part. (Figures of the living polypes of Flustræ, Wond. p. 605, pl. vi. fig. 6, 7.)
Many species of Flustræ occur in the British strata: the encrusting forms are attached to echinites, shells, &c.; the foliaceous are imbedded in chalk, sand, sandstone, &c. In Mr. Morris's Cat. Brit. Foss. ten species are enumerated; none of these are from formations below the Chalk. I have selected for illustration a Flustra attached to an echinite from Lewes. [Lign. 89], fig. 5, represents a small portion of the natural size; and fig. 4, a few cells magnified, to show their form and arrangement. A foliaceous zoophyte, apparently a bryozoon, is abundant in the Sussex and Kentish chalk, and is often disposed in angular folds. It is generally of a ferruginous colour, and, from its friable texture, it is probable the original consisted of a membranous polypidom or calcareous substance; specimens sometimes extend over several square inches of the chalk. It is common in the chalk-pit at Off ham, near Lewes.[251]
[251] In my South Down Fossils, pl. xv. fig. 6, a specimen of this kind is described as a Ventriculite, V. quadrangularis. An admirable lignograph of a remarkable example is given by Mr. Toulmin Smith, under the name of Brachiolites angularis; it presents ten deep, flat, angular folds, and has radicle and lateral processes; see "On the Ventriculidæ," p. 93.
Eschara.[252]—In these zoophytes the polyparium is encrusting or foliaceous, calcareous and brittle; the cells are thickened on their outer margins, and have a small, depressed, round aperture. They are arranged in two series of planes, adhering together, the cells on each surface exactly corresponding.
[252] So named from a supposed resemblance to an eschar.
Species of Escharæ are found either in flints, or attached to echinites, and other bodies; they have the appearance of patches of flustræ, but with a lens may be distinguished by the symmetrical juxtaposition of the cells on the opposite sides of the polyparium.