Zoology. — “On the Occurrence of desmas or desmoids in 
Hymeniacidon sanguinea”. By Prof. G. C. J. Vosmarr. 
(Communicated in the meeting of January 29, 1916). 
The term desma was first applied by Sorras (1887, p. 416) to 
the well-known irregular spicules of Lituistips. A desma is com- 
posed of two distinct elements, the crepis and the prosthema'). The 
crepis is a spicule, tetraxon or monaxon in form, and produced in 
a single mother-celi. It soon undergoes an arrest of development 
and the axial filament is entirely shut in by spicopal. On the crepis 
as foundation secondary layers of silica are deposited. These layers 
are at first concentric with it, but subsequently grow out into irre- 
gular branches, cladi, and tubercles which are altogether indepen- 
dent of it (Sorras 1888 p. LIX). Sorzas sometimes saw cells which 
resemble the ordinary scleroblasts situated in close contact with 
such a crepis and feels inclined to consider them as the mother-cells 
which seerete the epirhabd. Mincuin, whose early death we all 
learned with profound regret, was more definite. He wrote (1909 
p. 220) that the crepis “is produced in a single mother-cell. On the 
crepis secondary layers of silica are deposited by other cells”. This 
is plain enough. However, as far as [ know, nothing has been published 
on the subject after Sorras. The question is of importance and it 
is highly desirable that arguments should been given which either prove 
or disprove SorLas’ suggestion. For the moment it is not decided. 
Whether formed by the scleroblast of the erepis or by other cells, 
the prosthema may at any rate be considered as a secondary for- 
mation of spicopal, since the axial canal of the crepis is shut and 
normal primary growth of the spicule therefore excluded. The 
crepis is usually considered as a spienle. If this is of a tetraxon 
nature the desma will become tetracrepid; if it is a monaxon 
rhabdocrepid (monocrepid). In several cases the original axial fila- 
ment cannot be seen; such desmas are called acrepid. 
The diagrams in fig. 1—38 explain the different parts. 
As a rule the desmas are considered to be characteristic of 
Lithistids. But Scamipr found similar spicules in certain other sponges. 
The question is in how far these are real desmas. 
Oscar Scumipt described in 1862 two “new species” of Suberites, 
which he called S. crambe and S. fruticosus. | showed in 1880 
1 meóeSvur, what is added, any addition. | propose this term prosthema for 
secondary additions of spicopal in general. In desmas the prosthema can represent 
either the epirhabd or the epactines of SorLas. 
75 
Proceedings Royal Acad. Amsterdam. Vol. XVIII. 
