1172 
in desmas or desmoids it happens as irregular knobs and tubercles, 
are questions I am as little- prepared to answer as why some spicules 
are monaxon, others tetraxon etc. I only wish to draw attention 
to what I believe to be analogous phenomena. 
In describing the development of desmas in Neosiphonia superstes, 
Sollas says explicitly that cladi and tubercles are formed independently 
of the axial thread. But he continues (1888 p. 300): An axial 
portion, however, is still to be traced through the twigs and branches. 
It consists of silica of different refractive index and different solu- 
bility in the outer coatings, and runs as a wide core...” ete. This” 
is, however, by no means peculiar to desmas. It has been long 
known that a lamellar structure is often met with in sponges. 
Birscut1 demonstrated that different layers may show different 
refraction, an observation which Wijsman and I myseif afterwards 
confirmed. Leaving out of discussion the explanation, it may be 
stated that in most spicules the different layers not only have a 
different refractive index but also a different solubility. I have made 
in this matter a number of observations, which I hope to continue. 
These observations all point to the fact that we have to do with 
very complicated, partly optical phenomena. Roughly speaking we 
can say that the spicopal around the axial thread has a lower 
refractive index than the peripheral layer or layers *). Similarly is 
the solubility in bydrofluorie acid of the central layers is stronger than 
that of the pheripheral ones, in so far as the former are easier dissol- 
ved. Another difference between the layers is observed after careful 
heating; the well known brownish colour first appears in the inner 
layers and seldom occurs in the most external layer just under the | 
spicule-sheath. All these phenomena are seen in desmoids of /7. sanguinea 
just as distinctly as in simple styli of this or other Sponges. 
LIST OF REFERENCES. 
(chronological). 
1826 (;) Grant, in: Edinb. new Phil. Journ. I. 
1842 Jounston, A History of British Sponges. 
1862 Scumipt, Die Spongien des adriatischen Meeres 
1864 Körriker, Icones histolog. I. 
1) Even very slight differences in refraction are easily seen by a bluish or 
pinkish colour. If spicules are observed in a medium, the index of which differs 
but slightly from the spicopal, a pinkish colour proves that the spicopa! has a 
slightly lower refractive index than the medium. On the other hand the index of 
the spicopal is slightly higher than the medium if it appears with a bluish tinge. 
Spicules seldom disappear entirely in the medium, because the spicopal is almost 
invariably built up in layers of different refractive index. 
