SPONGES 



19 



Using a probe (a wire with knob at end, or small hat pin), try 

 to trace the canals from the pores to the cavities inside. 



Do the fibres of the sponge appear to 

 interlace, or join, according to any system ? Jp\ 

 Do you see any fringe-like growths on the 

 surface which show that new tubes are be- 

 ginning to form? Was the sponge growing 

 faster at the top, on the sides, or near the 

 bottom ? 



Burn a bit of the sponge; from the odour, 

 what would you judge of its composition ? " 2 ' — a P on 2 e * 



Is the inner cavity more conspicuous in a simple sponge or in a 

 compound sponge like the bath sponge? Is the bath sponge 





Fig. 27. —Bath Sponge. 



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Fig. 28. — Bath Sponge. 



branched or lobed ? Compare a number of specimens (Figs. 26, 

 27, 28) and decide whether the common sponge has a typical 



shape. What features do their forms 

 possess in common? 



Sponges are divided into three classes, 

 according as their skeletons are flinty 

 (silicious), limy (calcareous), or horny. 



Some of the silicious sponges have 

 skeletons that resemble spun glass in 

 their delicacy. Flint is chemically nearly 

 the same as glass. The skeleton shown 

 in Fig. 29 is that of a glass sponge which 

 lives near the Philippine Islands. 



The horny sponges do not have spi- 

 cules in their skeletons, as the flinty and 

 limy sponges have, but the skeleton 

 glass sponge. is composed of interweaving fibres of 



