DENDY— CALCAREOUS SPONGES 85 



triradiates, and huge longitudinally placed oxea, the latter lying for the most 

 part on the inner side of the layer of triradiates. The tubar skeleton is of the 

 inarticulate type (Figs. 8<^, 8e), composed of the centrifugal rays of the subgastral 

 triradiates and the centripetal rays of the subdermal triradiates (Fig. 8e, o^). 

 The former usually extend outwards right through the chamber layer, while the 

 latter may extend inwards as far as the gastral cortex or may fall considerably 

 short of this. Just within the oscidum the stout oral rays of the sagittal gastral 

 triradiates, here distinctly alate, are extended parallel with the oscular margin 

 and packed close together in a dense feltwork. Immediately on the inner side 

 of this feltwork lie the hair-like oxea of the oscular fringe. The oscular fringe 

 thus arises from the gastral cortex and the colossal dermal oxea take no part 

 in its formation. 



The spicules may be grouped under the following heads : — 



1. Dermal triradiates (Fig. 8a). These are nearly equiangular but sagittal owing 

 to the greater length of the basal ray. The rays are usually straight and rather 

 slender, tapering gradually to sharp points, the basal ray being a good deal 

 longer and rather more slender than the orals. In a tjrpical example the oral 

 rays measured about 0-11 by 0-01 mm. and the basal about 0-24 by 0-008 mm. 

 There -is a good deal of irregularity in the arrangement of these spicules, but 

 typically the basal ray points away from the osculum as usual. 



2. Gastral triradiates (Fig. 8&). These are a good deal larger than the dermal 

 triradiates and more markedly sagittal, the basal ray being usually very long 

 and slender, while the oral rays are often more or less curved backwards. The 

 oral rays are sometimes very unequally developed, one being much longer than the 

 other. The rays are slender and more or less gradually sharp-pointed. In a 

 typical example the oral rays measured 0-2 by 0-012 mm. and 0-17 by 0-012 mm. 

 respectively, and the basal ray 0-44 by 0-012 mm. The basal ray is typically 

 directed away from the osculum as usual. Near the osculum the oral rays are 

 curved backwards so much as to lie almost in a hne with one another, and are 

 much more strongly developed than the basal. 



3. Gastral quadriradiates (Fig. 8c). These resemble the gastral triradiates, but 

 they are decidedly stouter. The apical ray may be very strongly developed 

 (Fig. 8c, a.r.), straight and sharply pointed, and directed obliquely upwards in the 

 gastral cavity. These spicules are not very numerous. 



4. Subgastral sagittal triradiates (Fig. 8d). These are typical alate spicules, 

 with slender, gradually sharp-pointed rays. The recurved oral rays are extended 

 almost in line with one another in the deeper part of the gastral cortex; the 

 straight basal ray extends through the chamber layer to the dermal cortex. In 

 a typical example the oral rays measured about 0-145 by 0-01 mm. ; the basal 

 about 0-245 by 0-01 mm. 



