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 

 together 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 



