( 646 ) 
The experiment can be modified in the following way. Isolated 
spicula are brought into acid fuchsine; if the hydrofluoric acid is 
now allowed to act on the spicules the spicopal will be dissolved, 
whereas the sheath and the central thread will be stained red. In 
both eases the silica is dissolved; in the former case the thread and 
the sheath are visible because they are black (carbonised), in the 
latter case because they are red. In the original experiment the 
spicopal is only optically dissolved. 
What has been said for the styli of Tethya holds true m.m. for other 
spicules of Demoterellida. The structure of several spicules — monaxons, 
tetraxons or polyaxons — is fundamentally the same; in details 
there are important differences. However, I do not wish to speak 
about them in this paper. I have only mentioned as much as seemed 
to be necessary to show that by the described methods we are able 
to demonstrate most plainly the central thread. This can be done 
also in those cases in which the thread is not visible under ordinary 
circuinstances, e.g. if the spicules are very minute or irregularities 
of the surface prevent it. Thus, for instance, in Tethya no central 
threads are visible in the oxyasters or at any rate they are not present 
beyond doubt’). If these spicules are heated with great precaution 
they look under the microscope like fig. 19. It depends, as in other 
spicules, on the grade on heating whether the thread will be blackened 
only or with it its surroundings. Independently of this it is evident 
that the axes originate from one point. 
Applying the heating method to the spicules in question of Spir- 
astrella bistellata (O. S.) Ldfd. the microscope reveals pictures as 
drawn in fig. 1—8. It is most evident that we have here an axis 
exactly like that which unquestionable spinispirae possess. Such images 
are entirely unexplainable if the spicules are considered as congrescences 
of two euasters. They fully exhibit their true nature of spicules 
belonging to my group of «-spiraxons (1902 p. 112). Although I 
suppose this to be convincing, I applied moreover the dissolving 
method. It seems rather a paradox that the shape and the structure 
of a siliceous spicule can be cleared up by dissolving the silica. 
Still it is a fact, as I have frequently learned. Wijsman and myself 
(1905 p. 18) confirmed Bürscuarrs observations of 1901, that the 
dissolution of spicopal may proceed in more than one way. Only 
we have given another explanation of the fact. According to our 
1) On the whole spongiologists speak about the central thread as a constant 
feature of spicules. As a matter of fact stands that the presence of a thread is 
proved only in some cases and that in numerous microscleres nobody saw it, 
As far as | know only KOLLIKER found it in oxyasters of Tethya (1864, Pl. IX, fig. 2). 
