OF THE SPONGIAD^E. 53 



are simultaneously projected, as in Figs. 177 and 178, and 

 the result is a regular rectangulated quadriradiate form, as 

 in Fig. 181. Or if the second ray be at a right angle to 

 the one first projected, the result is an irregular quadrira- 

 diate figure, as represented by Fig. 180. In like manner 

 the irregular pentradiate form arises from the absence of 

 one of the four secondary rays, as in Fig. 182; or it some- 

 times occurs that the apical portion of the inequi-acerate 

 axial spiculum is deficient, and the result is, as represented 

 by Fig. 183, a regular pentradiate form. If the whole 

 of the radii are equally produced, the result is then the 

 regular attenuated rectangulated hexradiate spiculum, 

 (Fig. 174.) 



Sometimes, but rarely, we find a single ray more or 

 less spinous at its distal end ; in this case it is probable 

 that it was attached by that point to the membranous 

 structure, or to some part of the keratode of the 

 skeleton. 



The whole of these interesting spicula were obtained 

 from Mr. Cuming's specimen of Euplectella aspergiUum, 

 Owen. They are abundant in that sponge, frequently 

 filling up the interstices of the network of the siliceous 

 skeleton, or otherwise entangled in the tissues. hi 

 Dr. A. Farre's specimen of Euplectella cucumer, Owen, 

 they are equally abundant, and are not to be distinguished 

 from those in Mr. Cuming's specimen. They are, like 

 the great external prehensile spicula, and the fibre of 

 the skeleton, composed of numerous concentric layers of 

 silex, which readily separate from each other by decom- 

 position. 



I cannot say with absolute certainty that this tribe 

 of spicula belong really to the sarcode, as I have never seen 

 specimens of either of the species I have named, in which 

 they occur in profusion, in such a state of preservation 

 as to allow of their position being positively determined ; 

 but as in another specimen of sponge with a siliceous 

 skeleton like that of Dactylocalyx pumicea, Stutchbury, 

 the sarcode is preserved in excellent condition, and occurs 

 in such abundance, filling all the interstices of the skeleton 



