The inhabitants of the colony are then united in great numbers under one common envelope; these are longish filaments of the size of a swan's feather, reminding one of the appearance of the silk thread known by embroiderers as chenille. The downy appearance is produced by the collection of tentacula belonging to this curious swarm. The filamentous mass is the translucent row of cells in which these animalcules are lodged, and to which they retreat when disturbed. These cells are sometimes free in part, sometimes completely rooted to the stems of aquatic plants. The tentacles are of a fine transparent glass colour, the body being of a brown colour. Fig. 122 represents Cristatella mucedo, which is common both in this country and in France.
Most naturalists have now agreed to place among the Bryozoa certain species of animalcules which long remained imperfectly known. Amongst these are the Flustra, the Eschara, and other ascidians.
The Flustra are marine Bryozoa, whose skin in hardening forms a thin shell of horny or cellular appearance; their little cells, more or less horny, are grouped symmetrically, somewhat like the cells in a bee-hive. Sometimes they form a crust which covers the algæ and other submarine bodies; sometimes they form ribbon-like stems. In some species the cells are only found on one side; in others they occupy both. Their orifices are extremely small, and defended by spines quite microscopic (Fig. 123).
Their tentacles are covered with cilia, always vibratile, disposed in a straight line, which in their movements produce the effect which a row of animated pearls might be supposed to produce if rolled upwards from the base to the summit of the organ.
Fig. 123. Flustra foliacea (Linnæus).
The Eschara form leaf-like expansions, the entrance to their cells having also their protecting spines.
The expansions still represent microscopic bee-hives, the inhabitants of which enjoy at once a common and an independent existence. As it is with the corals, so it is here; each eats for the benefit of itself and for the community. Labour and nutrition for the community, labour and food for itself.