( 647 ) 
conception the spicopal which limits the central canal is more easily 
dissolved than that of subsequent layers. It seems that this is likewise 
the case for the radii of oxyasters of Tethya. It is probable that this 
depends on a difference in the quantity of water the “gel” contains. 
Now we observed that in pointed undamaged spicules, where conse- 
quently the central canal is shut, the funnelshaped dissolution is not 
seen, at any rate not at the very beginning of the process. The apex 
simply becomes thinner and thinner till the dissolving agent reaches 
the neighbourhood of the central canal, in which case the “funnel” 
often appears. Consequently we have herein another method to prove 
the existence of a central canal. 
On the other hand we may conclude from this, that, if a funnel 
never appears there is no central canal resp. no special layer of 
spicopal in the centre. Thus, for example, in spicules with spines, 
the latter disappear gradually and the spicule becomes gradually 
thinner. [ have observed this phenomenon very distinctly in acan- 
thostyli of an Zctyon from Naples. 
If the latter method is now applied on Spirastrella bistellata (O.S.) 
Ldfd. we see, that the pointed processes of the spinispirae become 
thinner and shorter, and finally disappear whereas the rest of the 
spiculum later becomes thinner (fig. 4—18). The microscopical images 
one sees during this process leave no doubt with regard to their 
structure. The more the spines dissolve, the more it becomes evident 
that we have to do with spinispirae. 
Moreover, it follows from the above experiments that the spines of 
these spinispirae are of quite another nature than the actines of the 
Tethya-asters. In the former case (Spirastrella) we have to do with 
local extuberances of spicopal destitute of any central thread or 
canal. In the latter case (Tethya) we have organic axes. Indeed, 
the former spicules are monaxons, the latter are polyaxons. 
Consequently the microsclera of Spirastrella bistellata (O.S.) Ldfd. 
are indeed spinispirae. Since LENDENFELD, TorseNr and myself believe 
to have found sponges, which are identical with Tethya bistellata 
of Oscar Scumipt, the species belongs as little to Hymedesmia as to 
Tethya. For the moment there is not sufficient evidence not to bring 
it to Spirastrella. The name for Tethya bistellata O.S. has to be, 
therefore, Spirastrella bistellata (O.S.) Ldfd. I believe with Torsexr 
that it is identical with Spirastrella cunctatriz O.S. 
44 
Proceedings Royal Acad. Amsterdam. Vol. XI. 
