528 



STRUCTURE OF SPONGES 



-SPICULES. 



the walls of special chambers lying at some distance beneath 

 the surface ; and these communicate with a system of Canals by 

 which the whole fabric of the Sponge is traversed. These Canals, 

 which are very irregular in their distribution, may be said to 

 commence in the small pores of the surface, and to terminate 

 in the large vents ; and a current of water, maintained by the 

 action of the cilia lining the chambers, is continually entering at 

 the former, and passing-forth from the latter, during the whole 

 life of the Sponge, bringing-in alimentary particles and oxygen, 

 and carrying-out excrementitious matter. 



406. The Skeleton which gives shape and substance to the mass 

 of sarcode -particles that constitutes the living animal, is composed, 

 in the Sponges with which we are most familiar, of an irregular 

 reticulation of Horny fibres. The arrangement of these may be 

 best made -out, by cutting thin slices of a piece of Sponge sub- 

 mitted to firm compression, and viewing these slices, mounted 

 upon a dark ground, with a low magnifying power, under incident 

 light. Such sections, thus illuminated, are not merely striking 

 objects, but serve to show, very characteristically, the general dis- 

 position of the larger canals and of the smaller areola with which 



they communicate. 



Fig. 272. 



In the ordinary 

 Sponge, the fibrous 

 skeleton is almost 

 entirely destitute of 

 Spicules, the absence 

 of which, in fact, is 

 one important con- 

 dition of that flexi- 

 bility and compres- 

 sibility on which its 

 uses depend. When 

 Spicules exist in con- 

 nection with such a 

 skeleton, they are 

 either altogether im- 

 bedded in the fibres, 

 or they are im- 

 planted into them 

 at their bases, as 

 shown in Fig. 272. 

 In the curious and 

 beautiful Dactylo- 

 calix pumiceus of 

 Barbadoes, however, 

 the entire network of 

 fibres is composed of 

 Silex, and is so transparent that it looks as if composed of spun 

 glass. The same is the case in the wonderful Euplectella of the 



Portion of Halichondria (?) from Madagascar, 

 with spicules projecting from the fibrous net- 

 work. 



