SPICULA OF SPONGES. 



113 



character. These spicules are of a wonderful variety and elegance 

 of form, for their shapes are not only strictly determinate for 

 each species of sponge, but each part of the sponge, it is believed, 

 has spicula of a character peculiar to itself. Sometimes they 

 are pointed at both ends, sometimes at one only, or one or both 

 ends may be furnished with a head like that of a pin, or may 

 carry three or more diverging points, which sometimes curve 

 back so as to form hooks. Sometimes they are triradiate, some- 

 times stellar, in some cases smooth, in others beset with smaller 

 spinous projections like the lance of the saw-fish. As they are 

 generally composed of flint, it may well be imagined that our 

 household sponge entirely owes its value to their absence in 

 its highly flexible structure. The sponge-skeleton is covered 

 externally and along the internal surfaces of the canals with a 

 gelatinous or slimy substance, similar to that which constitutes 

 the body of the Rhizopod, and which, seemingly inert and unor- 

 ganised, is yet the seat of whatever life the sponge contains. It 

 is by this slime, which may be pressed out with the finger, that 

 the net-work is deposited, and from it the whole growth of 

 the mass proceeds. 



On examining a sponge, the holes with which the substance 

 is everywhere pierced may be seen to be of two kinds: one 

 of larger size than the rest, few in number, and opening into 

 wide channels and tunnels which pierce the sponge through 



Kalina papillaris. 



Currents passing inwards through the pores (a.a), traversing the internal canals (6), and 

 escaping by the larger vents (c.d). 



its centre; the other minute, extremely numerous, covering 

 the wide surface, and communicating with the innumerable 

 branching passages which make up the body of the skeleton. 



