* 



I 





THE SPONGES. 



85 





obliquely longitudinal direction through the axis or branch, from one 

 flattened face to the opposite (Fig. 2, Plate 9). Or, instead of such 

 perforating canal-like spaces, the branch may exhibit elongated clefts 



(Fig. 2 



Plate 9, Fig. 3, Plate 10), which likewise pass through from 



The two structures probably belong in 



one flattened face to the other. 



the same category. 



The stony skeleton is the dictyonal framework of the sponge. This 

 consists of a close reticulum (Fig. 11, Plate 9), which is not divisible 

 into lamellae. In the peripheral region of the sponge, beams directed 

 more or less radially to the surface, with intervening connectives, may be 

 distinguished. The radial beams commonly terminate at the surface in 

 bosses or spines, which are sometimes short and conical, more often slender 

 and finger-like. The superficial bosses, like the beams themselves, are 

 smooth. The meshes have rounded corners, are irregular in shape, but 

 in the peripheral region tend toward a quadrilateral outline. The nodes 

 of the reticulum are for the most part not conspicuously thickened, 

 but sometimes they are. The thickness of the beams is very commonly 



about 50 fi. 

 . In places the beams of the dictyonal framework are so arranged as 



to give the framework a fibrous character. The fibres which lie in the 

 long axis of the stem or branch are visible here and there over the general 

 surface. They may also be traced with some distinctness in longitudinal 

 sections, which are cut either parallel or at right angles to the flattened 

 faces of the skeleton. The fibres are distinct on the walls of the large 

 canals or slits which perforate the sponge, and here they are especially 

 conspicuous on the walls which are parallel to the edges of the flattened 

 branches, and less conspicuous on the walls which are parallel to the faces 

 of these branches. The longitudinal fibres are especially distinct in some 

 of the very small and thin branches, Fig. 4, Plate 9, where they arch 

 outward toward the surface of the branch. 



Spinose hexacts (Fig. 10, Plate 9) are scattered in considerable num- 

 ber through the dictyonal framework. They are free, or fused with one 

 another, at points where the rays cross. The rays are slender and beset 

 with small, sharp spines, which are sometimes obsolete. In the same 

 spicule the rays often differ in length, but a common size is about 100 /x 

 Ion a- x 4-5 /x thick. The ends of the rays are pointed, or sometimes 

 enlarged and rounded. 



