RADIATES. 350 
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 
oe ll 
ean ® 
big. 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- 
