THE SPONGES. 117 



Skeletal Arrangement. 



The surface of the body, exclusive of the oscular papillae, is densely 

 covered with the small tylostyles (spicule 3), which project about 80 \l. 

 The spicules are arranged in diverging tufts ("bouquets" of Topsent), but 

 the bouquets are so densely set that this arrangement is not obvious in 

 surface view (Fig. 1, Plate 21). The spicules project obliquely over the 

 pore areas, and tend to obscure the boundaries of these areas (Fig. 1, 

 Plate 21). 



The cortex beneath the layer of surface bouquets is filled with a felt- 

 work of spicules, Fig. 6, Plate 16, chiefly the large tylostyles (spicule 2). 

 This cortical layer is about 500 /x thick, and the greater part of the spicules 

 forming it lie more or less tangentially. 



From the interior, near the centre of the attached surface, stout skeletal 

 bundles radiate toward the surface, Fig. 6, Plate 16. A typical bundle, 

 midway in its course, measures 400 [l in thickness. The chief spicule in 

 the radial bundles is the style (spicule 1). At the outer end of the bundle 

 where it passes into the layer of surface bouquets, the constituent spicules 



of the bundle, which are arranged lengthwise in it, diverge somewhat. 



In the choanosome between the radiating bundles are some scattered larger 

 spicules, and a considerable number of the small tylostyles. 



The outer surface of the oscular papillae (Fig. 4, Plate 16), like the 

 general surface, is densely covered with the small tylostyles, which here 

 project about 160 /z ; spicules arranged in closely set bouquets. The inner, 

 cloacal surface of the papillae is armed with a sparse layer of small tylo- 

 styles, which project radially or obliquely for a short distance into the 

 cloacal cavity. Stout bundles of large spicules ascend vertically in the 

 wall, lying about in its middle. A compact cortical layer of spicules, such 

 as occurs elsewhere in the body, is absent, although large tylostyles are 

 scattered in various positions through the papillar wall. 



