8 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



those spicules that have canals the polarizer shows plainly two distinct 

 areas of crystallization, the outer one being the more perfect. In those 

 spicules that have no separate replacement of the canal, the whole 

 spicule is replaced by amorphous silica, generally of an opalescent hue. 



The globo-stellates and other spicules of the dermal layer present a 

 somewhat different appearance. In many cases these Spicules are 

 entirely perfect, the globule, the spine, and the minute barbs being 

 perfect in all their microscopic details. The color of these spicules 

 varies from a light brown to a dark yellow, all more or less transparent, 

 and some of them quite so. In polarized light these have the same 

 properties as the amorphous silica, with the exception of color and a 

 higher single refraction. One case was found where the globo-stellate 

 had been replaced by crystalline chalcedonic silica, but it was imperfectly 

 preserved, and the barbs on the spines were so short and poorly finished 

 that it was not possible to figure it. This fact clearly shows us that we 

 must suppose either that the spicules have been replaced by amorphous 

 silica, or that they are still in the hyaline or colloidal state as formed, 

 and colored yellow by some organic agent perhaps. While this latter 

 supposition seems most reasonable from the fa«ts considered, yet if such 

 be the case it would seem probable that some spicules would be found 

 partly replaced with crystalline silica, a phenomenon which was not 

 discovered. However, before we admit this doubt to its full value, it is 

 necessary to remember that the field of observation was extremely 

 limited, and that such may exist in sufficient number to show all Btagea 

 of replacement. I shall designate this transparent yellow condition of 

 the spicules as colloidal silica, for want of a better term, at the same time 

 recognizing the fact that it has changed considerably in color from the 

 original condition in which it was secreted by the animal. 



While the preservation of many of the globo-stellates is perfect, even 

 to the showing of the minute spines and barbs in the proper relation to 

 the globate centre, yet some of them appear in a more or less fragmental 

 condition, which reveals an interesting fact in their history. Some show 

 the spines and barbs bent slightly toward the central body, as in Figure 

 lL'; in others the barbs are broken off or otherwise removed, and are 

 missing entirely ; while in others the globate portion is broken and part 

 of it removed, as in Figure 13. How much of this breakage and removal 

 is due to mechanical force it is difficult to say, fur there is evidence that 

 at least part of it is due to solution. It appears that the globulai centre 

 is dissolved more quickly and easily than the spines, hut it, is plain that 

 the spines will yield first to any mechanical force. Sollas found by 



