780 MR. A. G. THACKER ON [Juiie 16, 



The Skeleton consists of the following elements : — (1) Rather 

 small triradiates of the parenchyma ; (2) small dermal triradiates ; 

 (3) gastral quadriradiates and triradiates ; (4) large oxea, four 

 times as thick as the parenchymal triradiates; (5) minute oxea ; 

 and (6) long slender oxea of the oscular fringe. 



The main mass of the skeleton is composed of parenchymal 

 triradiates (text-fig. 166, a). These are subregular in shape 

 and their rays are almost always undulated ; the length of the 

 rays varies from 0'12 mm. to 0*22 mm., and the maximum thick- 

 ness from 0"015 to 0-024 mm. They are sharply pointed, A few 

 of these spicules sometimes develop apical rays of variable size 

 and shape, thus becoming quadriradiates. 



The dermal triradiates are similar to those of the parenchyma, 

 .but are smaller ; they rarely exceed 0"1 mm. in length or 0"01 mm. 

 in diameter. 



Text-fig. 166. 



Spicules from Leiicandra gemmipara. 



a — Parenchymal triradiate X 120. h = Gastral quadriradiate X 120. 

 c = Large oxeotes X 24. d = Small oxeotes X 120. 



The central gastral cavity and a great part of the canal-system 

 are lined by sagittal quadriradiates and triradiates (text- 

 fig. 166, b). The wide-spreading lateral rays are usually slightly 

 shorter than the basal ray, and are either straight or slightly 

 curved ; all the rays are sharply pointed. The apical ray is 

 curved at its extremity. A typical quadriradiate gave the follow- 

 ing measurements : — Length of lateral rays 0'12 mm. ; length of 

 basal ray 0*15 mm.; length of apical ray 0*06 mm. ; maximum 

 thickness of facial rays 0"012 mm., and of the apical ray somewhat 

 less. There are some triradiate spicules of similar form. 



ISTumerous large oxea project from the outer surface of the 

 sponge. They issue from the dermal cortex at a very acute angle 

 with the surface and their projecting ends all point towards the 

 osculum. They are spindle-shaped and are slightly curved, and 

 gradually but sharply pointed at both ends (text-fig. 166, c). Fully 

 half the length of the spicule is usually projecting beyond the 



