560 STRUCTURE AND ACTIONS OF SPONGE. 



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 magnify- 

 ing power, it is perceived that they form a complete 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 framework is composed of 

 these tubular fibres, which in some parts are set very closely 

 together, whilst in others they are loosely arranged. They con- 

 sist 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 meet with in the 

 Vegetable Kingdom; and it is to their elasticity, that the Sponge 

 owes those properties, which make it so useful to Man. 



1140. In other forms of the group, however, spicula of earthy 

 matter are disposed amongst these ; which add to the firmness of 

 the structure, and diminish its elasticity, according to the amount 

 in which they are present. These spicula are in some instances 

 composed of carbonate of lime; in others they consist of pure silex; 

 and their form varies according to the material. The earthy 

 spicula are disposed at intervals through the whole mass ; but 

 they are especially abundant in the neighbourhood of the canals, 

 and around the external orifices, both of these and of the pores. 

 In fact each of these orifices is strengthened by a regular frame- 

 work of spicula disposed around it ; and a high magnifying 

 power enables us to see, that across the entrance to the pores, 

 there is also a very delicate membranous network, which acts as 

 a filter or sieve, preventing the entrance of improper particles 

 along with the fluid which is drawn in through them. In many 

 instances, the sharp ends of the spicula project from the surface 

 of the Sponge, as if for its defence. 



1141. When Sponges are examined in their living state and 

 natural condition, a constant and rapid stream of water is seen 

 to issue from the larger orifices or vents. This stream is made 

 apparent by the movement of the minute particles contained in 



