THE SPONGIDA. 



15 



Fig. 4. A, Hypothetical section of a Spongilla ; a, superficial layer; 6, inhalent aper- 

 tures ; c, ciliated chambers ; d, an exhalent aperture ; e, deeper substance of the 

 sponge. The arrows indicate the direction of the currents. B, A small sponge with 

 a single exhalent aperture, seen from above (after Lieberkuhn") ; a, inhalent apertures ; 

 c, ciliated chambers ; d, exhalent aperture. C, A ciliated chamber. D, A free- 

 swimming ciliated embryo. 



