432 EVENINGS AT THE MICROSCOPE 



skeleton is not a continuous covering by which the sarcode 

 is invested, but consists of fibres or points or rods of vary- 

 ing form, which are clothed with the sarcode. This loose 

 sort of skeleton may be of horny or chitinous matter, like 

 that of Arcella., or calcareous, like that of the Forami- 

 nifera, or it may be siliceous that is, composed of flint 

 (silex). 



In some cases, as in the common Turkey Sponge, the 

 horny skeleton consists of a network of solid but slender 

 fibres, very tough and elastic, which branch and anasto- 

 mose in every direction, at very short intervals, as you 

 may see by looking at this atom which I cut off from a 

 dressing sponge. 



In the lime and flint Sponges, however, the continuity 

 and cohesion of the skeleton does not depend upon the 

 organic union of the constituent parts, as it does in the 

 loose and open network of the Turkey Sponge. For it is 

 made up of an immense multitude of glassy needles, all 

 separate and independent, between themselves, yet so con- 

 trived that they do hold together very firmly, and in a 

 great number of cases are arranged on a prescribed plan, 

 so as to give a certain form and outline to the ag- 

 gregate. 



If you have ever shaken up a box of dressing-pins, and 

 have then endeavored to take one out,, you know how by 

 their mere interlacement they adhere together in a mass, 

 so that by taking hold of one you may lift a bristling 

 group of scores. Somewhat on the same principle are the 

 calcareous and siliceous pins (spicula) of a Sponge held 

 together by mutual interlacement. Yet their cohesion is 

 aided by the tenacity of the living sarcode which in- 

 vests them; for I have found that specimens of Grantia 



