The function of these curious appendages is partly to warn off trespassers and partly to capture and retain small animals till decomposition has set in; in the latter case, the currents set up by the tentacles draw in the particles to the mouths of the polypides. The avicularia have arisen by modification of the ordinary cells, in which the muscles have developed at the expense of the degenerated polypides, the cells have become much smaller, of different shape, and separated out from the rest; the mandible represents the lid or operculum of the ordinary cell. The avicularia vary greatly in size and shape in the different genera; in _Flustra_, for instance, these organs closely resemble the ordinary cells. [Illustration: Fig. 7. _Bugula bicornis._ Cells magnified. (After Busk.) ] [Sidenote: Case A. Upright part.] In _Bugula bicornis_(*) (Fig. 7), from 1950 fathoms in the Southern Indian Ocean, each cell is provided with two avicularia with remarkably long stalks. The graceful vase-shaped _Kinetoskias cyathus_(*) (Fig. 8), one of the treasures of the “Challenger” Expedition, was dredged from 1525 fathoms off Cape St. Vincent. The stem, which tapers gradually upwards, rises from a tuft of root fibres. The cup is formed of slender branches supported at the base by a delicate membrane. The branches are composed of biserial rows of cells (Fig. 9) opening towards the interior of the cup. The avicularia are pear-shaped and pedunclate. Probably, in life, the cup is capable of being opened out to a considerable extent. Specimens of this species were also obtained from 2160 fathoms in the South Atlantic. [Illustration: Fig. 9. _Kinetoskias cyathus._ A branch magnified. _a_, an avicularium. (After Busk.) ] [Sidenote: Case A 1.] _Scrupocellaria reptans_, or the Creeping Coralline (Fig. 10 A, B) forms branching colonies, creeping over rocks and seaweeds, and attached by horny fibres often provided with curved hooks. The branches are composed of cells arranged in a double row. Each cell has the membranous area of its front surface protected by a branched flattened spine or operculum, and is produced and narrowed below; at the upper outer margin is a minute triangular avicularium. At the base of the back surface is a small sack-shaped cell with a cleft at the upper end, in which a horny bristle is articulated. The little cell is termed a vibracular cell, and the bristle a vibraculum.[21] This organ has arisen by a further modification of an avicularium, whereby the horny lid of the latter has become a long bristle. The bristles by their motion keep off intruders, and possibly act as scavengers by sweeping the surface of the cells. [Illustration: Fig. 8. _Kinetoskias cyathus._ (From Voy. Challenger, Atlantic: Wyv. Thomson.) ] [Illustration: Fig. 10. _Scrupocellaria reptans._ A. Creeping over seaweed, natural size; B. Front surface, magnified.