CALCAREA. 13 



It is unfortunate that this interesting structure cannot be made out with certainty. 

 The origin of the flagellated chambers near the oscule can be seen clearly ; an isolated 

 group of flagellated cells forms outside the gastral layer, and over this a few triradiates 

 and oxea grow (see Fig. 31a). As the flagellated chamber grows it tips up the 

 triradiates and oxea, which thus begin to point outwards while they continue to grow 

 (see Fig. 316). At the oscular end of the sponge therefore the flagellated chambers 

 do not grow as diverticula of the gastral layer, nor is there any lining of. flagellated 

 cells in this part of the gastral cavity. 



Spicules (Fig. 32). 

 The Oxea are of one sort : 



[a) Small projecting oxea, slightly irregularly bent, faintly hastate at the 

 thinner end, bluntly pointed at the thicker end. 70 m to 210 ju long x 6 m 

 to lOjU thick. 



Triradiates. The triradiates are of two sorts : — 



(c) Alate triradiates from the gastral layer. Basal ray straight, tapering 

 slightly, then pointed ; maximum size 480 /u long and 12 /x thick. Paired 

 rays unequal, slightly irregularly bent, maximum size 160 m long x 12 /-t 

 thick. Oral angle 130° to 135°. 



{d) Alate triradiates from the tubar skeleton. Basal ray straight, tapering 

 slightly, then pointed, 100m to 140m long x 6m to 7 fji thick. Paired 

 rays nearly equal, straight, 70 m long x 6 m to 8 m thick. Oral angle 120° 

 to 130°. 



The Quadriradiates are of one sort : — 



[e) Alate quadriradiates from the gastral layer. Basal ray straight, tapering 

 slightly, then bluntly pointed, 320m to 480m long x 10 m to 12m thick. 

 Paired rays unequal, nearly straight, 140 m to 240 m long x 10 m to 12 m 

 thick. Oral angle 130° to 140°. Apical ray bluntly pointed, slightly bent 

 orally, 130 m to 180 m long, oval in section, 8 m to 12 m thick x 12m to 

 16 m deep. In side view the apical rays sometimes appear swollen 

 near the point, the depth being greater than at the junction with the 

 facial rays. 



Oscular spicules : — 



(/) Quadriradiates from the oscular edge, basal ray straight or slightly bent 

 backwards, 110m to 140m long x 8m thick. Paired rays about equal, 

 bending downwards, bluntly pointed, 80m to 110m long x 10 m thick. 

 Oral angle 145° to 155°. Apical rays short and sharply pointed. 



