THE PORIFERA. 



Ill 



'"''m^m^^'"' 



Fig. 12. — A. Hypothetical section of a SpongiTIa: a. superficial layer; b, fnhnlent 

 apertures ; c, ciliated chambers ; d, an esh^lent aperture ; e, deeper substance 

 of the sponge. The arrows indicate the direction of t'le currents. B. Kf>^msiWSpon- 

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

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

 chamber. D. A free-swimming ciliated embryo. 



