RADIATES. 



353 



They consist wholly of a substance which is considered, 

 from the smell produced by burning it, to be much like 

 the horny substance found in many animals. There are 

 two kinds of pores a vast number of minute pores, and 

 here and there larger ones among them, termed vents. 

 Examined in their living state, it is manifest that from 

 the larger pores of the Sponges water is constantly pass- 

 ing out in currents, and it is supposed that it as constant- 

 ly passes in through the minute pores. This is analogous 

 to some movements that occur in certain animals. The 

 net-work of which sponge is composed is found, by ex- 

 amination with the microscope, to be made up of fine 

 tubes. One hundred and fifty different species have been 

 described by Lamarck. 



617. In Fig. 274 is a representation of a section of a 



Fig. 274. Section of living Sponge. 



piece of sponge, exhibiting the branches which conduct 

 the water from the minute interstices to the large vents. 

 The currents which come out from these vents are ren- 

 dered apparent by the minute particles of matter which 

 happen to be in them, as represented in the figure. The 

 Sponge lives on the water and what the water holds in 

 solution, and for its growth it is therefore necessary that 

 water should be constantly circulating through it in the 

 manner which I have described. There is one species in 

 which, the Sponge being of the shape of a bottle, the ab- 



