55 



These oxyliexacfcines, wliicli can, in consequence of tlieir large size be considered 

 as principalia, form a single layer on each side of the main skeleton-net. Their 

 rays are vertical to each other, two opposite ones being situated longitudinally, 

 parallel to the axis of the sponge. The oxyhexactines measure 400-500 f* in 

 diameter and their rays are round, smooth, straight or slightly curved, near the 

 centre 8-10 M thick and gradually attenuated towards the sharp-pointed end (pi. 

 X, f. 7). 



The hypodermalia are stout, sword-shaped hexactines and resemble in their 

 general character and in the thickness of their rays the parenchymal oxyhex- 

 actines described above. They differ from them however in that their rays are 

 unequal in length and terminally roughened. This roughness is often very 

 slight ; it is most clearly marked on the outer, radial ray, which protrudes beyond 

 the surface, where it occasionally extends right down to the vicinity of the 

 centre of the spicule. This distal ray and the four paratangential rays which 

 lie below the dermal membrane are about 250 m long. The proximal radial ray, 

 which points inward and penetrates the parenchyme, is 1000-1200 i^ long and 

 its pointed and roughened end nearly reaches the main skeleton-net. 



The hypogastralia are similar to the hypodermalia but much shorter and 

 destitute of a proximal, radial ray. They are accordingly, as in all other known 

 species of Euplectella, pentactlnes (pi. X, f. 7). It is to be remarked that the 

 four transverse (basal) rays which lie in, or just below, the gastral membrane, are 

 in most regions, if not everywhere, slightly cm^ved, concave towards the body of 

 the sponge-tube-wall. 



The main skeleton-net lies considerably nearer to the inner than to the outer 

 surface of the tube-wall, the parenchyme overlying it on the outer dermal side 

 being twice as thick as that overlying it on the inner, gastral side. In the super- 

 ficial layer of the outer parenchyme graphiocoms, with branch-ray-brushes 200 /* 

 long and 16 /^ thick (pi. X, f. 11) are met with. 



As a rule one floricom is attached to the protruding, distal ray of each sword- 

 shaped hypodermal oxyhexactine. The floricoms bear on each of their 6 main- 

 rays 7-9 (usually 8) branch-rays, arranged like the calyx of a flower. The 

 thickened terminal discs of the branch-rays have about 8 fairly stout marginal 

 teeth on the outer border (pi. X, f. 8-10). The whole floricom measures on an 

 average 120 /^ in diameter. 



The basalia-spicules of the root-tuft arise from the longitudinal spicular 

 fibres of the main skeleton-net and are several centimeters long. Two kinds of 

 basalia, both anchor-shaped but essentially different, are to be distinguished. 

 The basalia, of the first kind are smooth-rayed pentactines which have the shape 

 of tetradentate anchors. One of their rays, the main ray, which forms the 

 anchor-shaft is situated longitudinally, directed upwards, very long and pointed 

 at the distal, upper end. The four other rays, which form the anchor-teeth, are 



