575 



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

 fluenced by the laws of animal organization only, and never oy 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 . Spicnla 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, 

 according to the necessities arising from the mode of the construction 

 of the skeleton. 



2nd. The connecting spicula are not necessarily a part of the 

 skeleton; they are a subsidiary portion of it under especial circum- 

 stances, in a few genera only, as Geodia, Pachymatisma, and other 

 sponges which have a thick crustaceous surface, which the spicula 

 serve to support and retain in due connexion with the mass of the 

 animal beneath. The normal form of these spicula is very different 

 from that of the general mass of those of the skeleton, and they are 

 much more complex and varied in their structure. They usually have 

 a long, stout, cylindrical or attenuating shaft terminating either 

 acutely or hemispherically at the base, while the apex is divided into 

 three equi-angular radii, which assume in different species a con- 

 siderable amount of variety as regards form and direction. The tri- 

 radiate apices are usually cemented firmly to the inner surface of the 



