PLATE U. 



A FRESH-WATER SPONGE, 



A. Hypothetical section of a Spongilla : a, superficial layer ; b, inhalent 

 apertures ; c, ciliated chambers ; d, an exhalent aperture ; e, deeper sub- 

 stance of the sponge. The arrows indicate the direction of the currents. 

 B. A small Spongilla with a single exhalent aperture, seen from above : 

 <z, inhalent apertures ; b c, ciliated chambers ; </, exhalent aperture. C. 

 A ciliated chamber. D. A free-swimming ciliated embryo. 



