574 



the oldest- established and best-known type of the genus, and to refer 

 all others that may distinctly differ from that type to new genera 

 founded on structural principles. 



It is proposed to characterize the elementary tissues in the follow- 

 ing order : 



1. Spicula. 



2. Keratode or horny substance. 



3. Membranous tissues. 



4. Fibrous tissues. 



5. Cellular tissues. 



6. Sarcode. 



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, 



