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EXPLANATION OF PLATE XIX. 



Fig. 1. A view of part of a section of a small specimen of Grantia compressa at right 

 angles to the axis of the sponge, exhibiting a longitudinal section of the ciliated 

 cells. 



o. The exterior surface of the sponge. 



b. The interior surface. 



c. The cell in which the cilia in situ were observed in action, and in which 



the cellules on which the cilia are apparently based were in the best 

 state of preservation. 

 Fig. 2. Represents a portiou of the inner surface of a small Grantia compressa. The 

 central group of three diaphragms (a) exhibits the appearance of the cilia when 

 slowly in motion within the diaphragm, and at b, c, the same appearance of the 

 cilia, but with extraneous matter adhering to their apices. 

 Fig. 3. A group of detached tessellated cells, seen by a power of 1120 linear. 

 Fig. 4. Detached tessellated cells and cilia. 



a. Cilium in a state of inaction. 



b. Cilium in the position when about to strike towards the diaphragm. 



