Mr. J. S. Bowerbank on the Anatomy of the Spongiadse. 399 



And, in the second place, to treat of the organization and physio^ 

 logy in the following order : — 



1 . The skeleton. 



2. The sarcodous system. 



3. The interstitial canals. 



4. The intermarginal cavities. 



5. The dermal membrane. 



6. The pores. 



7. The oscula. 



8. Inhalation and exhalation. 



9. Nutrition. 



10. Cilia and ciliary action. 



1 1 . Reproduction, gemmules, &c. 



And to conclude with observations on the generic characters. 



The author then proceeds to describe the spicula, which he states 

 are essentially different in character from the fibres of the sponge ; 

 although the latter may be equally siliceous with the former. How- 

 ever closely the spicula may be brought into contact with each other, 

 or with siliceous fibre, they appear never to unite or anastomose ; 

 while the fibre, whether siliceous or keratose, always anastomoses 

 when it comes in contact with other parts of its own body or with 

 those of its own species. A detailed description is given of the 

 origin and progressive development of these organs, from which it 

 is inferred that they are the homologues of the bones in the higher 

 classes of animals, and that the forms they assume are always of an 

 organic type, never crystalline or angular ; and the same forms of 

 spicula are found composed of either silex or carbonate of lime, 

 demonstrating the fact that the deposits of earthy matter are in- 

 fluenced by the laws of animal organization only, and never by those 

 of inorganic or crystalline arrangement. 



Each species of sponge has, not one form of spiculum only, equally 

 dispersed throughout its whole substance ; but, on the contrary, 

 separate parts have their appropriate forms ; and thus we find that 

 there are often three, four, or even more forms of spicula in the 

 same individual. The author therefore, in describing them, pro- 

 poses to treat of these organs in the following order : — 



1 . Spicula of the skeleton. 



2. Connecting spicula. 



3. Defensive spicula. 



4. Spicula of the membranes. 

 .5. Spicula of the sarcode. 



6. Spicula of the gemmules. 



1st. The spicula of the skeleton in the siliceous sponges are 

 usually simple, elongate in form, slightly curved, and are occasion- 

 ally more or less furnished with spines. They are either irregularly 

 matted together, collected in fasciculi, or dispersed within or upon 

 the keratose fibres of which the skeleton is to a great extent com- 

 posed. All these elongate forms of spicula are subject to extreme 

 variety of length. In some species they maintain a great degree of 

 uniformity, while in others they vary to a very considerable extent, 



