Sub-order 2.—Ctenostomata.
Case B 2, and A upright part.
The Ctenostomata are fleshy, horny, or membranous; never calcareous. When the tentacles of a polypide are retracted into the cell, they are protected above by a membranous comb-like frill.
The cells either bud off from each other or arise as buds on a stolon or stem.
Case A, upright part.
Alcyonidium gelatinosum(*) (Fig. 17), so called from its resemblance to the zoophyte Alcyonium, forms fleshy translucent growths occurring in the form of nodulated branched masses, or of long finger-like growths. The species is common round our coasts where it grows attached to stones and shells near low-water mark.
Fig. 17.
Alcyonidium gelatinosum. A, a small piece, natural size; B, the same magnified.
Case A, upright part.
Amathia forms bushy growths composed of slender horny branches. The cells, which are cylindrical or squarish, rise from the branches in biserial rows like Pan’s pipes. In Amathia lendigera(*) (Fig. 18) the groups of cells are well separated from each other, but in A. spiralis(*) and A. convoluta(*) the cells form a nearly or entirely continuous series winding in a spiral round the slender stems. In Bowerbankia imbricata(*) the cells are clustered on the stems. Vesicularia spinosa(*), or the Silk Coralline, forms delicate brown tufts resembling a filamentous alga; the cells arise separately in a single series from the hollow tubular stems and are contracted at their point of attachment.
Fig. 18.
Amathia lendigera. A, natural size; B, magnified.
Nearly all the Ctenostomata are marine, but a few species live in fresh water.