1170 
likeness to the microcalthrops of Plakina trilopha. They are for the 
greater part found in groups on the same substratum as the sponge 
975. They are kept together bv traces of degenerated soft tissue. 
Now it is certainly of importance that in several cases I found on 
the same stone together with the red inerustations mentioned above 
crusts of Plakinidae. The majority of my specimens of Plakinidae 
came from the same grounds as the red crusts of H. sanguinea viz. 
Posillipo, Pozzuoli and Nisida. The supposition that the irregular 
siliceous bodies are to be derived from spicules of Plakinidae seems 
to me, therefore, not over fantastic. We know by F. E. Scsunzr’s 
researches how variable the spicules of Plakinidae are. The various 
shapes of the desmoids we find sometimes in Hymeniacidon sanguinea 
are all easily explained, if we admit that spicules of Plakinidae 
form the crépis.. We might suppose that little crusts of these curious 
Porifera are overgrown by the stronger, expanding Hymeniacidon 
and are finally killed by it. In this way groups of spicules of some 
Plakina or other may be incorporated in the parenchyma of Hy- 
meniacidon. But, these spicules are not yet desmas or desmoids. 
We saw that, according to Sorras, Mincuin and others, desmas are 
formed by secondary deposit of silica on a spiculum, which is early 
arrested in growth,and by which process the axial thread becomes shut 
off from the surrounding cytoplasm of the scleroblast. Whether the 
spicopal, which will form the prosthema is secreted by the mother- 
scleroblast of the crepis or by others, is of no consequence for 
our suggestion. 
Is there anything to be seen in our Hymeniacidon which resembles 
the development and structure of true desmas? I believe there is, 
as far as can be judged from examination of objects no more in contact 
with their mother-cells. Every phenomenon we are able to observe 
in true desmas can be seen in the siliceous bodies under conside- 
ration. If we apply the heating method and subsequent mounting in 
glycerine, in general in observing the spicules in question in media 
of various refractive index, we get pictures which fully correspond 
to SorLras’s statements about desmas. So far there seems to be no 
reason for a distinction between desma and desmoid, since it is „ot 
confirmed in any way that the secondary silica is deposited by other 
cells than the mother-scleroblast of the crepis. Consequently, if one 
wishes still to make a distinction, it must be for other reasons. 
Such a reason might be found in the fact that the desmas of Lithis- 
tids are spicules normally secreted by the Sponge itself — at least 
as far as we know. But the desmoids of Hymeniacidon sanguinea, 
according to my views, do not belong to the Sponge; the erepis at 
