126 THE SPONGES. 



Skeletal Arrangement. Coarse columns or bundles of spiculo-fibres (Z. I. 

 in Fig. 7, Plate 16, and Fig. 2, Plate 17) extend more, or less vertically 

 through the body, branching as they go. The branches extend obliquely 

 upward and outward, and terminate as axial bundles in the conuli (Fig. 13, 

 Plate 17). The bundles are well seen in a piece of sponge that has 

 been macerated for some hours in cold caustic potash. 



The skeletal network, connecting the columns together and extending 

 between them and the surface, is on the whole irregular. But more or 

 less radially directed fibres, extending out from the branching columns to 

 the surface, may everywhere be distinguished. In some parts of the body 

 (Fig. 2, Plate 17) and invariably in the conuli (Fig. 13, Plate 17) they 

 are conspicuously developed, with connectives commonly about at right 

 angles ; the skeleton here becoming fairly regular. Elsewhere the radial 

 fibres are sparsely developed (Fig. 7, Plate 16), and are accompanied by 

 or pass into other strong fibres, which pursue a very oblique or tangential 

 course. The connectives in such regions are without order, and the 

 network is irregular. 



The vertical columns have not clearly defined boundaries, but their 

 thickness is in the neighborhood of 0.5 to 1.0 mm. They are composed 

 of coarse spiculo-fibres, 70-110 /x thick, together with finer fibres, all 

 interlacing to form a close irregular network (Z. h. in Fig. 7, Plate 16, 

 Fig. 2, Plate 17). Here and there in the constituent fibres of the 

 column, the spicules are less compactly arranged than elsewhere, and the 

 fibres themselves merge into one another. In such spots the reticular 



nature of the columns is only vaguely apparent. The stronger fibres 



of the general skeletal reticulum of the body, whether radial, oblique, or 

 tangential, are 50-80 /j, thick. The finer fibres or connectives measure 

 15-30 /x in thickness. In the conuli the radial fibres are slenderer than 

 in the body, about 30-40 /i, thick. The ultimate meshes of the skeletal 

 network, as may be seen from the figures, vary a good deal in diameter ; 

 a common width being in the neighborhood of 300 /a. 



The strong fibres of the general network (Fig. 3, Plate 17) and 

 of the vertical columns are well filled with spicules ; spongin, however, 

 abundant and forming a distinct sheath for the fibre. In the connectives 

 (Figs. 4, 5, Plate 17) the spongin is relatively more abundant and 

 the spicules are not as closely packed as in the larger fibres, sometimes 

 forming only an axial core. In the finest connectives, the spicules are 



