PROTOZOA AND SPONGES 435 



(calcareous Sponges with needles of three rays), when 

 long macerated in water, so that the sarcode is dis- 

 solved, 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 color; and is always found 

 closely investing turbinate shells which are inhabited by 



SECTION OF SPONGE. 



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 colored fleshy gran- 

 ulesomitting for the present the skeleton. Of the 

 same substance is the whole slice composed, but looser 

 and more open as it recedes from the surface. It is 

 separated by blank spaces which are larger toward the 

 centre, smaller and more numerous as they approach the 



exterior. 



SCIENCE 19 



