ANIMAL ACTIVITIES. 
124 
The kind of sponge commonly called the hard-head 
The smaller 
holes on the outside correspond to openings in the 
ectoderm through which currents of water flow to the 
The large holes at the top are out- 
gives a general idea of sponge-structure. 
interior cavities. 
8, 
Pr 
AN 
\ 
A, section of sponge; 
&, part of a digestive sac; C, one cell from a digestive sac. 
ANS 
Fic. 108. — Structure of a Sponge (magnified). 
Along the passageways from 
lets for these currents. 
(oscula) there are enlargements which act like stomachs, 
that is, they take up the food as it passes along in the 
these outer holes (inhalent pores) to the larger holes 
The endoderm cells which line 
currents of water. 
