PORIFERA, OR SPONGE TRIBE. 559 



CLASS III. PORIFERA. 



1224. When we examine the exterior of a piece of common 

 Sponge, we observe that it is covered with minute orifices or 

 pores (whence the name which has been given to the class), 

 thickly set together ; and that larger openings are disposed at 

 intervals amongst these. If these larger orifices, or vents, be 

 traced into the substance, it will appear that they are the 

 mouths of canals or vessels which 

 ramify through it ; these have definite 

 walls formed by the firm tissue of the 

 sponge, but perforated with a large 

 number of orifices, connecting them 

 with the spaces that lie between their 

 net-work of branches. The pores, on 



FIG. 729. SPONGE. 



the other hand, open into a less regular 

 arrangement of small tubes and cells, of which the spongy 

 is principally composed. These spaces are produced by the in- 

 terlacement of the fibres that form the solid framework ; they 

 communicate with one another throughout the mass ; and the 

 canals seem to take their origin from the midst of them, arising 

 by small tubes which unite into larger ones, and these again 

 meeting to form the wide channels which terminate in the vents. 

 If, after making such an examination of the structure of the 

 Sponge, we tear a small portion of it into fragments, its fibrous 

 nature becomes very apparent. If we submit these fragments 

 to a low magnifying power, it is perceived that they form a com- 

 plete network, inosculating with each other in every direction. 

 By applying a higher power, it is ascertained that they are of 

 tubular structure. In the Common Sponge, the whole frame- 

 work is composed of these tubular fibres, which in some parts 

 are set very closely together, whilst in others they are loosely 

 arranged. They consist entirely of an organic texture, rather 

 approaching the horny substance of Animals (as is made evident 

 by the smell emitted on burning it), than anything which we 



