SYSTEMATIC ZOOLOGY FOR TEACHERS. 181 



even a colony of single-celled protozoans, for the multiplicity and differ- 

 entiation of cells is very apparent. 



General Characters. 



Sponges are animals of vegetable-like appearance, fixed when adult, free 

 when very young. *They are generally composed of two substances; ( 1 ), a 

 protoplasmic-like^ material which, although always soft, varies in firmness 

 from a jelly-like consistency to about the solidity of beef liver ; this is known 

 as sarcode, and is always present ; ( 2 ) a horny substance which forms an 

 inner skeleton,; but which is sometimes wholly absent, as in the fleshy 

 sponges. This horny skeleton is often supplimented by spicules of either sil- 

 ica or lime, or is more rarely wholly replaced by them. 



The sponge flesh, or sarcode, is penetrated by numerous tubes, usually 



"Water system of sponge. 



ramifying in all directions. (See fig. 83.) These are of three kinds and 

 each kind has a separate function, but all together form a water system. 

 One set, smaller than the others, open externally and are termed incurrent 

 tubes. Through these the water is taken into the interior of the sponge ; 

 (see fig. 83, a) here it is taken up by a second set of tubes, which are a 

 little larger than the incurrent, but which are continuous with them, and 

 conveyed to the third kind, which open on the outside, and are called excur- 

 rent. From them the water is thrown out. 



