386 



THE INHABITANTS OF THE SEA. 



genus Grantia they consist of carbonate of lime. Though the 

 skeleton of most sponges is formed both of horny fibres and 

 of mineral spicules yet the proportions of these two component 

 parts vary considerably in different species. In the common 

 sponge, for instance, the fibrous skeleton is almost entirely 

 destitute of spicules, a circumstance to which it owes the 

 flexibility and softness that render it so useful to man, while 



they predominate in the 



HUgs - ~»M &&> ■ ■ i\. 



Halichondriae, and some- 

 times even, as in ,the 

 Grrantias, completely su- 

 persede the horny fabric. 

 On examining a sponge, 

 the holes with which the 

 substance is everywhere 



Mmuteportionof the surface of Tethea Cranium, Tyiprrerl mmr Vip appn +n 

 magnified, spicula projecting beyond the pierceo. may De seen to 

 surface. kg f ^ WQ ]^ n ^ B . one 



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

 wide channels and tunnels which pierce the sponge through its 

 centre; the other minute, extremely numerous, covering the 



f 



Halina papillaris. 



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

 by the larger vents (c, d). 



wide surface, and communicating with the innumerable branch- 

 ing passages which make up the body of the skeleton. Through 

 Che smaller openings or pores the circumambient water freely 

 enters the body of the sponge, passes through the smaller 

 canals, and, ultimately reaching the larger set of vessels, is 

 evolved through the larger apertures or oscula. Thus by a still 

 mysterious agency (for the presence of cilia has as yet been 



