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THE SPONGES. 



129 



Ilymerapliia Carter (the spicules of which are shown in Fig. 3, Plate 18), 

 differing in some few details from Ilymerapliia minima Topsent (1892, 



p. 114, Plate 11, Figs. 2-3) . 



Spicules. Strongyle (Fig. 8, Plate 17), smooth, cylindrical, distinctly 

 curved, ends evenly rounded off. Size, 360-380 /x x 24 /x. 



Skeletal Arrangement The wall of the fistula is supported by a dense 

 skeleton, which appears as a confused mass of strongyles (Fig. 4, Plate 

 18, a tangential section), arranged for the most part tangentially to 

 the surface of the sponge, but lying in all (tangential) directions, united 

 by considerable .spongin, and interrupted here and there by rounded gaps. 

 This mass of spicules may be regarded as forming a reticulum, which 

 consists of vaguely outlined fibres, or tracts bounding small rounded 

 meshes. The tracts are often 150-200 /x thick, the meshes somewhat less 

 in diameter than the thickness of the tracts. Individual tracts are trace- 

 able only for short distances. Spicules of a tract arranged loosely and 

 about lengthwise ; united together with considerable spongin. 



The dense skeleton occupies most of the thickness of the wall, extending 



— about 



nearly to the inner surface of the fistula. The innermost layer — 

 100 ul thick — of the fistular wall contains a good many canals and granular 

 cells, and comparatively little skeleton. What skeleton there is has the 

 shape of vaguely defined tracts about 200 /x thick, which resemble those 

 of the skeleton in general, but most of which pursue an approximately 

 longitudinal course for considerable distances. The longitudinal tracts, 

 together with similar connectives, give rise to a reticulum, the meshes 



of which are long and narrow. 



The dense skeleton extends quite to the outer surface of the fistula, but 



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in its outermost layer the reticular character is lost, the spicules here lying 

 side by side, in any particular region parallel to one another (Fig. 8, Plate 



17, surface view). 



In the region occupied by the dense skeletal reticulum, only the spicules 

 and spongin are discernible in the present state of the sponge, and although 

 maceration has undoubtedly gone on, there cannot in the natural condition 

 be much soft tissue in this part of the fistula. 



Comparative. As regards the shape of the spicules the fistulae here 

 described most closely approach Oceanapia singaporensis Carter. In the type 

 of this species (Phloeodictyon singaporense Carter, 1883, p. 326) the spicules 

 are of two forms : oxeas and strongyles, the latter about \ the length of 



17 



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