14
which we shall describe presently, but the glass coil or glass rope is a structure altogether new. As we have already seen, it was originally thought not to be a portion of the sponge at all, but the foot secretion of an Actinia. It is now more commonly regarded as an integrant part of the sponge itself, chiefly from the fact that we have recently become acquainted with several other sponges similarly provided with foot pieces or stalks serving as bases of attachment. We may, if you please, look upon the glass cords, as kind of gigantic spicules dipping down into the soft mud and ooze, and serving as an anchor. If we take a single filament of the cord and examine its structure, we find that it is not homogeneous as the thread of spun glass would be to which it has been compared, but that it is laminated,—composed of layers upon layer of silicious matter deposited round a central axis. This can be seen without much difficulty by breaking the glass fibre across in such a way as to produce an irregular splintered fracture, and examining the broken end under a low miscroscopic power. The appearance presented is shown in the sketch, taken from an object laid on the table, and in the original it is easy to count more than twenty layers entering into the composition of the fibre. Towards each extremity of a fibre the number of layers is fewer and the fibre consequently tapers off, and as the fibres themselves do not extend the whole length of the coil, this likewise tapers off in the end which is attached to the sponge, and appears there to be tough and fibrous and securely attached to the spicules and network forming the skeleton of the sponge mass.
Mr. Carter in his very interesting observations on the development of the fresh water spongilla has described how the spicula of that organism are formed. He has shown that they first appear as a delicate line enclosed and developed within an elongated sarcode cell, and that they grow rapidly by external additions until they attain their full dimensions, soon outgrowing the cell in which they first appear. There can be little doubt that the filaments of the glass rope are developed in some more or
