6 ANATOMY AND PHYSIOLOGY 



111 the early stage of their development the spicula 

 appear to consist of a double membrane, between which 

 the first layer of silex is secreted, and in this condition they 

 present an internal cavity approaching very nearly to the 

 size of their external diameter. In this state they readily 

 bend abruptly in any direction without breaking, as may be 

 seen in Pig. 247, Plate XI, which represents a porrecto- 

 ternate spiculum from the termination of one of the radial 

 lines of the skeleton at the surface of Tetkea cranium. 

 This spiculum has been considerably distorted by pressure 

 on the points of the rays at its apex. The deposit of the 

 silex is not continuous and homogeneous, but is produced 

 in successive concentric layers, which it would appear are, 

 at least for a period, equally secreted by the inner surface 

 of the outer membrane and the outer surface of the inner 

 one ; for we always find that as the development of the 

 spiculum progresses, the internal cavity gradually becomes 

 less, until finally it exists only as a central canal of very 

 minute diameter in comparison with that of the spiculum 

 itself. These stages of development may often be seen in 

 the spicula of young specimens of Spongilla fluviatilis, 

 especially in the spring, when they are growing rapidly. If 

 small fragments of the sponge be slightly charred in the 

 flame of a lamp, and then submitted to microscopical ex- 

 amination, the outer and inner membranes of the spicula 

 will readily be rendered visible (Figs. 248, 249, Plate XI) ; 

 in immature spicula the internal membrane is represented by 

 a dense black film of charcoal, as in Pig. 249, Plate XI ; 

 while in the mature ones the small central cavity is seen to 

 be lined by so thin a membrane as to afford by its charring 

 a slight brown tinge only to its walls (Pig. 248, Plate 

 XI). The concentric deposition of the layers of silex or 

 carbonate of lime in the spicula are also readily to be 

 seen (Pig. 250, Plate XI) in transverse fractures of almost 

 any large spiculum, either siliceous or calcareous, and they 

 present the same aspect as similar sections of either the pris- 

 matic cells of shell tissue or the spicula of a Gorgonia. The 

 amount of silex, and the manner of its deposition in the 

 spicula, is not the same under all circumstances. Where 



