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less analogous way, and that in the living state they are, like the silicious skeleton of the sponge itself, clothed with sarcode matter from which layer after layer of silicious material is deposited as long as the fibre continues to live and to grow.
If we now exame the coriaceous envelope of the glass coil, or polythoa, which can be most easily done in dried specimens after it has macerated for a few days in water, we can easily make out that it consists of two layers, an inner one closely connected with some of the fibres of the glass rope, and which has been stated by some observers to extend between and invest each individual fibre of the glass coil which forms its axis;—I am, however, doubtful if this is the case; it does not appear to be so in the large specimens on the table;—and an outer thicker one which is largely made up of small particles of sand, broken shells, minute foraminiferæ, and here and there a diatom, mixed up with the beautiful spicules secreted by the organism itself. At first sight it reminds one of the little tubes which the terebellas build up from sandy and shelly particles and which you often meet with cast up on the sea shore. Studded about on the bark are little wart-like projections with flattened crowns, having in the centre a small depression or orifice with little radiating grooves proceeding from it. If, in a macerated specimen, we make with a sharp knife a thin section from without inwards, taking care to include the central depression, and then examine it with a common lens, we shall observe, supposing our section to have been a tolerably successful one, much the appearance which you see drawn here, the original of which is on the table. We see that the innermost of the two layers of the bark passes under the little cup-like projection, all together forming the base of the included cavity; whilst the thicker external layer, separating from the inner one, is raised up, forming the wall and crown of the tubercle until it. reaches the central depression, where it turns inwards, forming a small funnel-shaped process which opens into the general cavity of the structure. This at once reminds us of the formation of the Actinozoa,
