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ANIMAL ACTIVITIES. 
The kind of sponge commonly called the hard-head 
gives a general idea of sponge-structure. The smaller 
holes on the outside correspond to openings in the 
ectoderm through which currents of water flow to the 
interior cavities. The large holes at the top are out- 
Fig. 108. —Structure of a Sponge ( magnified ). A. section of sponge; 
B, part of a digestive sac; C, one cell from a digestive sac. 
lets for these currents. Along the passageways from 
these outer holes (inhalent pores) to the larger holes 
(oscula) there are enlargements which act like stomachs, 
that is, they take up the food as it passes along in the 
currents of water. The endoderm cells which line 
