Exhib ition of Sponges. 



239 



method of investigation of the Sp>ongiadce, 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 Zoophyta, the Ttmicata, 

 and the Nudibrancliiate mollusca. 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 Tethea 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. 



