Exhibition of Sponges. 
239 
method of investigation of the Spongiadce, undertaken by 
Dr Bowerbank; and from his well-known microscopic ability, 
persevering industry, and accuracy as an observer, no one is 
better qualified for the task, nor more likely to do full justice 
to it. 
In the paper referred to, the early stage and development 
of spicula is described. " They appear to consist of a double 
membrane, between which the first layer of silex is secreted, 
and in this condition they present an internal cavity ap- 
proaching very nearly to the size of their external diameter. 
In this state they readily bend abruptly in any direction 
without breaking. An interesting fact, in regard to the 
animal nature of the sponge, is, that many forms of the 
spicula have their types in the more highly organised class 
of animals, and especially among the ZoopTiyta, the Tunicata^ 
and the Nudihrancliiate moUusca. The spicula are always 
of an organic type, and never crystalline or angular. Each 
species of sponge has not one form of spiculum only, equally 
dispersed throughout its whole substance, but, on the con- 
trary, separate parts have each its appropriate form, and 
three, four, or even more forms often occur in the same indi- 
vidual ; and in TetTiea cranium there are no less than seven 
distinct shapes. The spicula appropriate to particular parts 
of the sponge are uniform in their general characters 
throughout the whole of the Spongiadce^ and a great portion 
of them are so well characterised by their form as to enable 
the student, when once well acquainted with their peculiari- 
ties, to assign each readily to its proper place in the sponge." 
These organs are treated of in the following order: — 
1. Spicula of the skeleton. 4. Spicula of the membranes. 
2. Connecting spicula. 5. Spicula of the sarcode. 
3. Defensive spicula. 6. Spicula of the gemmules. 
The monograph is accompanied with four beautiful plates, 
containing upwards of two hundred figures of the different 
forms of spicula, with a distinct appellation for each form. 
The scientific world is therefore indebted to Dr Bowerbank 
for a new arrangement of the Spongiadoe^ with a precise ter- 
minology and nomenclature, founded on the anatomical 
structure and physiology of the species. 
