( 107 ) 
Considering the facts that these spicula are generally very small, 
and that consequently a microscope of very high power is wanted to 
understand the true shape, it is evidently not easy to determine the 
number of entire revolutions or parts of it; the same may be said 
of the pitch of the ,spiral’ — or rather of the screw helix. 
In order to obtain certainty about this I constructed wax models, 
the axis of which were screw helices of various length and various 
pitch, of course all drawn on the same circular cylinder. The dia- 
‘meter of the models I made in accordance to the relative size observed 
in the spicula. Such a set of models ought to be carefully studied 
im projection. This can be done by looking at them with one eye, 
or, which is far better, by studying the shadows of the models in 
various positions. These projections are then compared to the camera- 
drawings or microscopical projections of the spicula themselves. This 
method most clearly shows fs“ that the bow-shape can be obtained 
with models of less than one revolution; 2"¢ that the C- or S-shape 
can be obtained with models of more than 1'/, revolution. This 
depends both on the length and the pitch of the screw helix, as is 
shown by the following table *): 
of icone anes | 
Fiend PAN 10° 
2/5 Ung = C [S] (A)| C [Sl A | 
3/4 C [Ss] — Cs (A) | Ge Sota 
| 
5/e C (S) [A] | [Oe Ae 1S CAS 5 0 aN 
1 [C] (S) [A] | Cone Sy LAS) Cert ae Gt Sane | 
cae SRG ACI EN "Sr io, |LOG SIS Meer | 
11/, © A} © SA] GA 
11/s ==. (8) A Ce SAL On EEN 
11/, ne Ie VAG), (S) AS | CS 
B | — SIA — SB] A] — — A} — BA 
15/, | eh de 
2 En er ake a Sah Fee 
1) CG, S or A means: C-shape, S-shape or bow-shape distinct; ( ) means 
indistinct; [ | means very indistinct. A dash — means that the shape cannot be 
obtained with the wax model. 
