SPONGES. 179 



of these radiating tubes. This represents the condi- 

 tion in the commercial sponges ; certain cavities are 

 lined with cilia and are connected on the one hand 

 with the smaller tubes entering the whole surface of 

 the sponge, and on the other with the large tubes 

 opening at the top. These cilia cause the currents 

 above mentioned. Thus the sponge gets both food 

 and oxygen. 



Sponges (including, besides those already men- 

 tioned, silicious sponges, whose spicules are flinty) 

 constitute the branch Porifera. 



For a very interesting account of the gathering and 

 preparation of sponges for the market, read "Com- 

 mercial and Other Sponges" by Hyatt (No. III. in 

 " Guides for Science Teaching "). 



