76 HUXLEY, ON CLASSIFICATION. 
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Fig. 3. A, Hypothetical section of a Spongilla ; a, superficial layer ; 4, inhalent 
apertures; ¢, ciliated chambers; @, an exhalent aperture; ¢, deeper sub- 
stance of the sponge. The arrows indicate the direction of the currents. - 
_.. B, a small sponge with a single exhalent..aperture, seen from above (after 
‘ Lieberkiihn) ; @, inhalent apertures; ¢, ciliated ‘chambers ; d, exhalent 
. ‘aperture. (;, a ciliated chamber ; D, a free-swiifiming ciliated embryo. 
