26 
STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
of the circulation, through the stomach, and then out by the 
mouth. In this way each flagellated cell eats and drinks, living 
to itself. It also breathes, the water which conveys its food con- 
taining dissolved oxygen, which passes into the cell by osmosis. 
Thus with food for fuel, and oxygen to burn it, the cell is pro- 
vided with energy, which it expends in maintaining the water 
circulation, from which it obtains food and oxygen again. Though 
each cell lives its own life, yet the different cells all work more or 
less in unison; thus when they have taken enough food to 
satisfy their wants for the time, they frequently rest together to 
digest it : more or fewer of them cease to lash the water, the ostia 
and pores are closed by the contractile sphincters, which also seem 
to be in sympathy, and the circulation goes on feebly, or, for a 
time, altogether stops, to begin afresh when digestion is completed, 
and hunger urges the cells to renewed activity. Though the 
flagellated cells live each, as has been stated, their own life, yet 
it is no less true that each lives for the rest of the organism, as 
indeed must happen in all organised communities. The nutrition 
received is, under favourable circumstances, more than enough to 
make good the loss of substance involved in work, and the surplus 
leads to that increase in size, which is termed growth. 
. But to increase in size, there is in every individual a limit, 
which overpassed usually leads to division, and thus soon after the 
flagellated cell has passed its full size, a constriction makes its 
appearance transversely round the basal part, and extends inwards 
till the lower part of the base is completely severed from the rest ; 
in this way the single cell becomes two, one of which retains its 
original character, while the other resembles an amoeba, and 
making its way into the mesoderm lives a wandering life, possibly 
like a colourless blood corpuscle, serving as a food carrier to the 
rest of the organism. This is the origin of the third kind of cells 
which we mentioned as forming a part of the mesoderm. The 
splitting or fission of the flagellate cell is not always transverse ; 
