constitute a surface of constant mean curvature; it is evidently a figure of equiôÙlibôÙrium under certain special conditions of restraint. The cylindrical coil of the Spirillum, on the other hand, is a surface of constant mean curvature, and therefore of equiôÙlibôÙrium, as truly, and in the same sense, as the cylinder itself.
A very curious conformation is that of the vibratile ãcollar,ã found in Codosiga and the other ãChoanoflagellates,ã and which we also meet with in the ãcollar-cellsã which line the interior cavities of a sponge. Such collar-cells are always very minute, and the collar is constituted of a very delicate film, which shews an undulatory or rippling motion. It is a surface of revolution, and as it maintains itself in equiôÙlibôÙrium (though a somewhat unstable and fluctuating