PolystomciTa. 



29 



Characters of Sponges. The common toilet 

 sponge is a representative of a group of animals 

 whose affinities are not easily understood. On ex- 

 amination with a magnifying glass it will be found to 

 consist of irregularly branching and re-uniting threads 

 of a highly elastic material, so arranged that the inter- 

 spaces between the finer branches appear as pores or 

 canals, which, from the nature of their walls, freely 

 communicate with each other. 



On examining the surface of a spongy some large 

 holes will be seen, which, on being cut into, are found 

 to be the extremities of wide FIG. 17. 



spaces or tubes ; these divide 

 within the sponge-mass into 

 smaller canals, which again divide 

 and subdivide until finally they 

 end in the fine canals whose 

 terminations are the minute sur- 

 face pores between the superficial 

 fibres of the mass. The walls of 

 these spaces are themselves full 

 of small pores in the interstices of 

 the fibres which form the sub- 

 stance. This horny mass is really 

 the sponge skeleton, having the 

 same relation to the sponge 



animal that the SpicilleS Of Radio- Ascetta, a calcareous sponge 



larians bear to the soft parts of those creatures. 



We can most easily understand the nature of a 

 sponge animal by examining such simple forms of the 

 group as may be found encrusting sea-weeds or stones 

 on our own shores. These are nearly cylindrical, 



