On the "Glass-Rope" Hyalonema* 87 



throughout its entire length is minutely and delicately striated. 

 These striae are not superficial, they indicate the edges of, or 

 the intervals between, a series of extremely thin concentric 

 silicious layers of which the wall of the spicule is composed. 

 A distinct double line running down the centre marks the 

 central canal, so characteristic of all sponge spicules. A 

 transverse section, or an irregular fracture, shows distinctly 

 the concentric silicious layers (Plate, Fig. 9). One point 

 in connection with the central canal is important as establish- 

 ing a relation between the spicules of the coil and those of the 

 sponge-body. About the middle of the spicule, at its thickest 

 part, the canal sends out two transverse branches, or sometimes 

 four in the shape of a cross. These branches are extremely short, 

 only displacing slightly a few of the inner silicious layers ; the 

 outer layers gradually resume their straight course, so that no 

 bulging or distortion is to be seen on the surface of the thread, 

 and the point, where the branching occurs, can only be dis- 

 covered with difficulty by transmitted light, and with a magni- 

 fying power of 300 diameters. When the sponge is fresh, the 

 spicules of the coil are covered from end to end with an 

 organic him, and there are likewise films of sarcodic matter 

 between the silicious layers. This can be shown by heating 

 the spicule over a lamp, when the organic matter shows out 

 brown by the separation of free carbon. The spicules are stiff, 

 but somewhat elastic. When forcibly bent, and then freed, 

 they instantly resume the spiral curve which was stamped 

 upon them during their growth. The spicules on the outside of 

 the coil are all of the same size and length, but in the 

 interior they are shorter and finer, diminishing to an inch or 

 so in length and to a hair-like fineness, so that new spicules 

 are added to the coil from within. 



In good-sized specimens the sponge-body is from five to 

 six inches high, and about three inches in its widest diameter. 

 The surface is shaggy w^ith the ends of irregular bundles of 

 spicules, and is closely dotted over with small round openings 

 about a line across. Round each of these apertures there is a 

 stellate arrangement of ridges, the ridges round one opening- 

 running into those round the opening next it, so as to cover 

 the whole surface of the sponge with an irregular raised net- 

 work. In the dried specimens from which, as^ yet,, all our 

 information is derived, these ridges, and indeed the v/hole 

 substance of the sponge, consists simply of interwoven silicious 

 spicules of various forms, with a mere trace of organic matter 

 between them, cementing them loosely together. In most 

 sponges with silicious spicules, the skeleton is partly made up 

 of membranes, or threads, or granular masses of a horny sub- 

 stance. A net-work of horny fibres is the most common form, 



