386 



THE MICROSCOPE. 



sponges the tissue is strengthened and supported by 

 spines, spicula, of various forms ; and which, in some 

 species, are siliceous, and in others calcareous. The 

 minute pores, through which the water is imbibed, have 

 a fine transverse gelatinous network and projecting spicula, 

 for the purpose of excluding large animals cr noxious par- 

 ticles ; water incessantly enters into these pores, traverses 

 the cells or tubes, and is finally ejected from the larger 



i 2 



Fig. 211. 



1, A portion of Sponge, Halichondria simulans, showing siliceous spicula 

 imbedded in the sarcode matrix. 2, Spicula divested of its matrix. 



vents. But the pores of the sponge have not the power of 

 contracting and expanding, as Ellis supposed ; the water 

 is attracted to these openings by the action of instruments 

 of a very extraordinary nature (cilia), by which currents 

 are produced in the fluid, and propelled in the direction 

 required by the economy of the animal'. 



Mr. Bowerbank, in a paper on the " Structure and 

 Vitality of Spongiadce" states that sponges consist princi- 

 pally of sarcode, strengthened sometimes by a siliceous or 

 calcareous skeleton, having remarkable reparative and 

 digestive powers, and consequently a most tenacious 

 vitality ; so much so, tfiat having cut a living sponge into 

 three segments, and reversed the position of the centre 

 piece, after the lapse of a moderate interval, a complete 

 junction of the parts became effected, so as to render the 

 previous separation indistinguishable. 



Professor Grant's careful and laborious researches, have 

 finally classed sponges in the animal series of the creation. 



