THE PORIFERA. 



Ill 



Fig. 12. — A. Hypothetical section of a Spongilla: a, superficial layer; ft, inhalent 

 apertures ; c, ciliated chambers ; <Z, an exhalent aperture ; e, deeper substance 

 of the soonge. The arrows indicate the direction of the currents. B. A small Spqn- 

 qllla with a single exhalent aperture, seen from above (after Lieberkiihn): a, in- 

 halent apertures ; c, ciliated chambers ; d, exhalent aperture. C. A ciliated 

 chamber. D. A free-swimming ciliated embryo. 



