
SPONGES. 451 
being moved in such a manner as to give rise to this discus- 
sion is in consequence of the action of the water in passing 
through them. According to the analysis of sponges by 
Hornemann, they consist of a substance "similar to osma- 
zone, animal mucus, fat oil, a substance soluble in water, a 
substance only soluble in potash, and traces of chloride of 
sodium, iodine, sulphur, phosphate of lime ( ?), silica, alu- 
mina, and magnesia.” The quantity of silica which consti- 
tutes the structure of sponges is remarkable. It generally 
occurs in the form of spicule in considerable quantities, 
embedded in the substance or body of the sponge. In the 
Species of Halichondria, the silicious spicule are pointed at 
the extremities, whilst the spicule of some are pointed at | 
one end only, and are round at the other; sometimes they 
appear cylindrical, curved, or straight. The spicule of the 
genus Pachymatisma are often sharp at one extremity and 
at the other expand into two points; some are sharp at one 
end and expand at the other into three points; the P. John- 
stoniæ can be taken as an example of the latter.. The genus 
Tethea possess silicious spicule having hooks at both ends, 
and amongst the genera Grantia, Geodia, and in the Levant 
Sponge, the spicule are very large and radiate into three di- 
rections like a three pointed star. When properly mounted 
they form very beautiful microscopic objects. The spicule 
of the Grantia nivea show them to be of the triradiate, or 
three pointed, star shape, those of the Halichondria Griffithii 
In the form of pins, whilst those of the common sponge, 
from the Philippine Islands, are sometimes in the shape 
of crutches or stars. In the common Madrepore Sponge 
(Dactylochalizx pumicea) the silicious element is fully devel- 
oped as the whole mass is composed of this extremely hard 
substance, which is disposed in tubular and radiating canals. 
One of the rarest, and I may say most beautiful of the sili- 
lous sponges, is the Zuplectella* speciosa Gray (Fig. T6). 
It is described in the “Transactions of the Zoólogical Society 
* Eu, well; and pleko, I weave. 
