20 RHIZOPODA. 



climate; some overspread the surface of the rocks 

 like living carpets, others expand in fan-like growths 

 of softest texture; some are cylindrical in shape, 

 while others emulate the forms of branching shrubs ; 

 others, again, are moulded into cups and giant gob- 

 lets, many festoon the walls of rocky caverns, or 

 depend, like living stalactites, from wave-worn roofs. 

 Examined with a microscope, however, a living 

 sponge is found to differ but little from the organisms 

 we have just been contemplating. No matter what 

 its form, the living portion of a sponge consists of a 

 soft slime that coats each fibre of its structure, and 

 this soft slime, when highly magnified, resolves itself 

 entirely into particles so like the Amoeba in their 

 characters and attributes, that they are evidently of 

 the same nature, the main distinction being that, 

 whereas in the case of the Foraminifera, they secrete 

 a calcareous shell, the sponges construct a common 

 framework, over which the liv- 

 ing film is spread. This frame- 

 work varies in its composition 

 in different kinds of sponge. 

 Sometimes it is made up of 

 tubes of horn, forming a net- 



FIG.S.-FRAMEWORK OF SPONGE. work i^^^d in all direc- 

 tions ; such is the common 



sponge of commerce, which owes its resiliency and its 

 capability of absorbing and retaining fluids, qualities 

 which render it so useful in domestic economy, to 

 the construction of its horny skeleton. Instead of 

 tubes of horn, the sponges usually found upon our 

 coasts deposit in their substance crystals of pure flint, 

 which vary much in form in different kinds, while a 

 third group strengthen their framework with calca- 

 reous spicula of variable shape. Three different 

 kinds of sponge may, therefore, grow close to each 

 other, bathed alike with the same sea-water, yet they 

 elaborate therefrom products so different as horn, 

 and flint, and lime, wherewith to build a fabric that 

 supports the whole community. On viewing a living 



