CALCAREA. 11 



munieating with the largest excurrent chamber in Fig. 18 is shown in Figs. 21-24 on a 

 larger scale. Fig. 21 represents the subgastral structure, and shows the cells grouped 

 round and partly over the excurrent chamber : the section includes the bottom 

 diaphragms and apopyles of five of the flagellated chambers. The excurrent chamber 

 extends below these apopyles, but exactly how far cannot ])e seen. The position of the 

 port in the gastral layer, which is much smaller than the full extent of the excurrent 

 chamber, is sho'WTi by the dotted line. Fig. 22 represents the next section higher, and 

 includes the top of the excurrent chamber with one apopyle ; the second apopyle 

 belonging to flagellated chaml)er K cannot be clearly made out and has not been 

 shown, though it must be present. Fig. 23 shows the same group of chambers higher 

 up and Fig. 24 higher still. 



Skeleton. — The gastral skeleton is a dense felt of large quadriradiates, with the 

 basal rays pointing in all directions, though the maj(jrity point more or less towards 

 the base of the sponge. The apical rays, which are very sharp, project into the gastral 

 cavity. The arrangement of the spicules is shown in Fig. 2G, which represents the 

 same port that is shown in Fig. 21 ; in this drawing only a certain number of the 

 spicules actually present are drawn. The large number of the distorted t spicules is 

 rather remarkable. The articulated tubar skeleton is formed of triradiates which are 

 lient over the distal end of the chamber, forming a thick rounded end and not a cone. 

 The tops of contiguous flagellated chambers are joined together by the fusion of their 

 skeletons, which thus foi'm a sort of dermal cortex, ef. Fig. 20, similar to that in genus 

 Dermaf retail. In the present species, however, the flagellated chambers are of different 

 lengths, and consequently the dermal cortex is very irregular and not clearly defined, 

 so that it seems advisal)lc to include it in Tenthrenodes rather than Dermatreton. 

 Round the distal ends of the chambers there are rings of oxea which project slightly. 



The oscular skeleton difi'ers very little from the normal gastral and dermal 

 skeletons. The quadriradiates are rather smaller than the ordinary gastral 

 quadriradiates. There is a thick fringe of hair oxea. The meshwork of flagellated 

 chambers extends right up to the oscular edge, terminating in a scolloped edge. 

 Fig. 25 represents a longitudinal section through the oscule at a point where the 

 mesh reaches the edge. 



Spicules (Fig. 27). 

 The Oxea are of one sort : 



{(i) Projecting oxea, 160 )u, to 240 m long X lOyu, to 15 /x thick, slightly and 

 iiTcgularly bent, thickest towards the outer end, which is bluntly pointed ; 

 the inner end is rather more sharply pointed. 



The Triradiates are of one sort : 



{h) Alate triradiates from the body-wall and dermal cortex. Basal rays 

 straight, tapering to. a sharp point, 90 ju. to 330 m long x 9 m thick. Paired 

 rays, unequal (appearing the more so owing to folding), bent slightly 



