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towards the eye, in a closed cell, as before. When thus viewed with 

 a quarter-inch combination, the sponge is seen to be composed of 

 angular cavities constructed of tri-radiate calcareous spicula, (PI. 

 XIX. fig. 2). They are closely packed together, like the cells of a 

 honeycomb. The cavities are of the same diameter downwards for 

 about the length of half their own diameter, and then they terminate 

 in a perforated diaphragm, the circular mouth of which is of about 

 half the diameter of the space above it, (PI. XIX. fig. 2). Beneath 

 this diaphragm there is an elongated cavity, or cell, the dome of 

 which, crowned by the diaphragm, is lined with tessellated cellular 

 structure, but which does not appear at the opposite extremity of 

 the cavity, or that in which the in-current orifices exist. 



The tessellated cells on the inner surface of the diaphragm are 

 disposed in lines, irregularly radiating from near its margin. 



Within this diaphragm, and between the inner termination of the 

 in-current orifices, are situated the cilia, which are of exceeding 

 tenuity, and of considerable length comparatively. 



If the diaphragm be focussed distinctly, and the sponge be alive 

 and active, we shall see, within the area of the circular orifice of the 

 diaphragm, the cilia in rapid motion, many of them being tipped 

 with a small portion of gelatinous matter, or of extraneous fecal mat- 

 ter, and the whole of them are continually oscillating, in a plane 

 parallel to the edge of the diaphragm ; while a current of water is as 

 continually being discharged through the orifice of each diaphragm, 

 into the short, angular cavities which line the interior surface of the 

 sponge. And this current is unmistakably apparent, from the fre- 

 quent discharge of minute molecules of extraneous or fecal matter. 

 I could not detect ciliary motion from the external surface of the 

 sponge, although they were in vigorous action within the cells, when 

 viewed from the opposite surface. Nor were tessellated cells or cilia 

 visible in any other portion of the same sponge. 



By this mode of examination the presence and action of the cilia 

 were very clearly demonstrated ; but I could not determine their 

 insertion, or the extent of surface covered by them. I therefore de- 

 cided, if possible, to get a section of the sponge at right angles to its 

 long axis, and endeavour to examine the whole length of the ciliated 

 cell, from its origin, immediately beneath the outer surface of the 

 sponge, to its termination, at the perforated diaphragm near the 

 inner surface ; but this was not easily achieved with so delicate and 

 fragile a subject. I found that sections made either with a knife or 

 with the most delicate scissors, had undergone so complete a disrup- 



