SEA-MATS AND SHELLY CORALLINES. 61 



the population of this tiny marine city. This, however, is 

 by no means a specimen of unusual size. 



These cells, which I compare to cradles, are of shallow 

 depth, but the head-part rises to a much greater height 

 than the foot. All round this elevated portion the margin 

 is armed with short blunt spines, two on each side, which 

 stand obliquely erect, projecting outwards over the middle 

 of the next cell, which thus, in concert with the spines of 

 the cell on the opposite side, they protect. 



If you search carefully over the mass of cells with this 

 pocket-lens, you will perceive that on some of them are 

 seated minute white globules, which look like tiny pearls. 

 These are not placed in any regular order, two being 

 sometimes found on contiguous cells, but generally they 

 are scattered at more or less remote intervals. If we now 

 apply the microscope to these appendages, each globule 

 is seen to be flat on that perpendicular side which faces the 

 foot of the cradle ; and this flat side is a movable door, 

 with a hinge along its lower edge. The door is of a 

 yellow hue ; the globule itself being, as I said, of a pearly 

 white hue. 



This is all that we can see in this dried specimen ; but 

 if we had been fortunate enough to examine it when first 

 it was torn from its attachment to an old shell at the 

 bottom of the sea, you would have seen much more. 

 And what would then have appeared I will describe to 

 you. 



Suppose, then, that a coverlid of transparent skin were 

 stretched over each cradle, from a little within the margin 

 all round, leaving a transverse opening just in the right 

 place, viz., over the pillow, and you would have exactly 

 what exists here. There is a crescent-form slit in the 

 membrane of the upper part of the cell, from which the 

 semicircular edge, or lip, can recede if pushed from ' 

 within. 



Suppose, yet again, that in every cradle there lies 



