386 EVENINGS AT THE MICROSCOPE. 



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

 of scores. Somewhat on the same principle are the cal- 

 careous and siliceous pins (spicula) of a Sponge held to- 

 gether by mutual interlacement. Yet their cohesion is 

 aided by the tenacity of the living sarcode which invests 

 them ; for I have found that specimens of Grantia (cal- 

 careous Sponges with needles of three rays), when long 

 macerated in water, so that the sarcode is dissolved, have 

 very slight power of cohesion among their spicula. 



To understand the structure of a sponge we will shave a 

 thin sectional slice from this Halichondria suberea. This 

 when alive is of an orange colour ; and is always found 

 closely investing turbinate, or top- shaped, shells which are 

 inhabited by Hermit-crabs. We will macerate the slice in 

 tepid water for a quarter of an hour, and then examine it 

 in the live-box. 



The surface is a thin layer of greater density than any 

 other part, and is composed of coloured fleshy granules, 



SECTION OF SPONGE. 



omitting for the present, the skeleton. Of the same sub- 

 stance is the whole slice composed, but looser and more 

 open as it recedes from the surface. It is separated by 



