682 



THE PORIFERA. 



many large cities are built wholly of the dead skeletons of these mi- 

 croscopic beings, and that in a single ounce of sand from the Caribbean 

 Sea nearly four millions of those shells have been discovered. 



The first sub-class of these beings is the Foraminifera, so called on 

 account of the tiny openings, or foramina, with which the pretty shells 

 are pierced. Sometimes, however, this shell is wanting, and its place 

 is supplied by a cover composed of matted sand-grains. 



PORIFERA. 



We now arrive at a large class of beings, which are by common con- 

 sent allowed to form the very lowest link in the animal chain. The 

 name Porifera is given to them because the whole of their surface is 

 pierced with holes of various dimensions, the greater number being 



extremely minute, 

 i^i\\^^ while others are of 

 considerable dimen- 

 sions. The well- 

 known Turkey 

 Sponge, so useful 

 for the toilet, will 

 afford a good ex- 

 ample of the porous 

 structure. 



The true living 

 being which consti- 

 tutes the Sponge is 

 of a soft and almost 

 gelatinous texture 

 to the unaided eye, 

 and with the aid 

 of the microscope is 

 found to consist of 

 _ an aggregation of 

 i separate bodies like 

 The Living Sponge. those of the Amoeba, 



some of which are furnished with long cilia. By the constant action 

 of the cilia a current of water is kept up, causing the liquid to enter 

 by innumerable pores with which the surface is pierced, and to be ex- 

 pelled through the larger orifices. A sponge in full action is a won- 

 derful sight, the cilia driving the water in ceaseless torrents, whirling 

 along all kinds of solid particles, arresting those which are useful for 

 digestion, and rejecting those with which it cannot assimilate. 



The extraordinary object which is called by the appropriate name of 



