The middle layer or mesogloea is not originally a cellular layer, but a gelatinoid structureless substance, secreted by the two cellular layers.
Gum arabic is not precipitated from solution by alum, stannous chloride, sulphate or nitrate of copper, or neutral lead acetate; with basic lead acetate it forms a white jelly, with ferric chloride it yields a stiff clear gelatinoid mass, and its solutions are also precipitated by borax.
The insoluble part of the gum is a calcium salt of bassorin (C12H20010), which is devoid of taste and smell, forms a gelatinoid mass with water, but by continued boiling is rendered soluble.
Both genera are characterized by their habit of secreting a tubular gelatinoid investment, the "coenoecium," composed of a number of superposed lamellae, doubtless the result of its intermittent secretion, mainly though perhaps not exclusively, by the proboscides of the zooids.