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had been replaced by crystalline chalcedonic silica, but it was imperfectly 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must suppose either that the spicules have been replaced by amorphous 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 formod, 
and colored yellow by some organic agent perhaps. While this latter 
supposition seems most reasonable from the faets considered, yet if such 
be the case it would seem probable that some spicules would be found 
partly replaced with erystalline 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 stages 
of replacement. I shall designate this transparent yellew 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-stéllates 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 
12; in others the barbs aro 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, for there is evidence that 
at least part of it is duo to solution. It appears that the globular centre 
is dissolved more quickly and easily than the spines, but it is plain that 
the spines will yield first to any mechanical force. Sollas found by 
