MARINE BIOLOGICAL STATION AT POUT ERIN. 



81 



Sycon ciliatum (1) and Sycon (or Grantia) com/pre ssum (2), 

 like many others, have microscopic spicules made of hard 

 chalk or carbonate of lime ; while in most of our sponges, 

 as in the crumb-of-bread sponge (Halichondria panicea), 

 shown at 3, the spicules are of silica or flint. This 

 character enables us to distinguish two important sets of 

 sponges, the Calcareous and the Siliceous. 



Fig. II., Sponges (natural size). 



Sponges were at first thought to be lifeless, then for 

 a time they were regarded as plants. Professor Grant, 

 after whom a common sponge was called " Grantia," first 

 showed that they were animals, and that while alive a 

 current of sea-water passes through the sponge — in by 

 minute pores all over the surface, and out by one or more 

 larger crater-like openings. 



