Potash on the Siliceous Skeletons of Sponges. 289 



The siliceous cement which, with the included spicules, 

 forms the skeletal fibre does not scale off under the action of 

 potash in concentric layers, as happens in the case of true spi- 

 cules, but dissolves away amorphously both in the interior and 

 on the exterior of the fibre, without in the latter case producing 

 those hemispherical pit-like markings which cover the exterior 

 of many deciduous recent and fossil spicules, and which in 

 some cases have been shown by Mr. Carter to result from the 

 depredations of some algoid parasite. 



Fine longitudinal striations, however, are generally obser- 

 vable along the fibre, and may perhaps indicate a lamellar 

 structure ; in one or two instances I have seen indications of 

 more rapid solution at a point [p) midway between the spicular 

 canal and the periphery of the fibre, thus (fig. 2), as though 

 along the line where the siliceous cement first covered over 

 the contained spicule. 



Fk. 3. 



The appearance shown in fig. 3 seems to result from a 

 change in the refractive index of the spicular component of 

 the fibre; for the included spicule (s) is quite distinctly defined 

 from the surrounding fibre and is yet composed of solid silica, 

 which opposes the expansion of a well-defined thread of air (a) 

 contained in its relatively narrow axial canal. 



Long before the spicular canals in tlie interior of the fibre 

 have become fully developed by the action of the potash, the 

 minute "rosettes" of the sarcode have completely disappeared 

 in solution, so that before stage 2 is passed a careful search 

 with high powers fails to reveal even the slightest trace of 

 them. 



The free sexradiates follow shortly after; but the spines of 

 the fibre remain for some time longer, and do not disappear 



