structure and life-history of a sponge. 25 
as we have already stated, in a powerful stream through the mouth. 
But as water is driven out of the radial tubes the pressure within 
them is diminished, and to restore equilibrium, — to supply the 
place of the water which has been expelled, — fresh water flows in 
from the interior through the pores at the ends of the tubes, and 
also, after passing down the inter-canals, through the pores at the 
sides of the tubes. 
The circulation thus established from the pores through the 
radial tubes into the stomach, and so out through the mouth, is 
the means through which, as we shall now see, the nourishment 
and respiration of the sponge are carried on. 
The water in which the sponge lives is inhabited by a large num- 
ber of infusoria and other minute forms of life, and contains 
besides many small particles derived from decaying organisms ; 
these enter the sponge, borne along with the inflowing currents of 
water, and are seized upon by the flagellate cells of the endoderm, 
as they pass through the radial tubes. 
The manner in which the flagellated cells extract their food from 
the water is worth noticing ; it is precisely similar to that in which 
the flagellated monads, which so closely resemble these cells, feed. 
No sooner does a little particle of food touch the edge of the 
delicate collar which surrounds the collum, than it adheres to it 
and is carried down by currents, that circulate up one side of the 
collar and down the other, to the end of the collum, in which, 
along with an accompanying drop of water, it becomes at once en- 
gulped. If the particle should come directly in contact with the 
collum itself, it is engulped in the same way. The included drop 
of water, enclosing its particle of food, travels down the collum 
into the base of the cell, where it forms a little ‘ bleb,’ which we 
have already noticed as a ‘ vacuole,’ The food of the vacuole 
undergoes digestion, and when all the ‘ goodness ’ has been got out 
of it, the indigestible residue is extruded from the cell, through an 
extemporised aperture, to be forthwith swept away in the torrent 
