— 
copic cells each having one lash. The cells are arranged in one 
layer on the walls ofthe chamber, and rapidly lashing the water 
in one direction, they are the cause of this constant stream of water. 
The inward channel terminates on one side of these chambers, and 
the outward channels begin on the other side, so the water is 
sucked in at the pores and driven out violently at the little 
mouths. The entering currents contain minute particles of 
food, and also Oxygen in solution, the outward stream contains 
Carbonic Acid and compounds of Nitrogen and Carbon and 
Hydrogen, such as are formed by all living animals. The cells 
will bear a still closer examination, they consist of a spherical body 
of protoplasm, granular within but firmer and _ clearer outside, 
containing a nucleus, and one or more contractile vesicles; one 
end rests against the wall of the chamber, and the other, the free 
end, is prolonged into its cavity with along clear neck; around 
the margin of the free end there is an extremely thin collar, and 
from its centre a long slender lash protrudes. The collar is a soft 
semi-liquid material which is constantly circulating, streaming up 
on the outside and down on the inside, like an endless band in 
machinery. Particles of food drawn to it by the lashing movement 
are carried up the outside and down the inside, till they reach the 
base, where they enter the substance of the cell; a little drop of 
water is included with them, and thus the cells not only eat but 
drink ; the constant circulation of the collar exposes a continual 
change of surface to the water by which Oxygen is absorbed, and 
so the cell breathes. 
The tissues ofa sponge consist of three layers, an outer, middle, and 
inner layer of cells. The outer consists of flattened cells with 
many angles, which cover the whole outside of the Sponge, 
and line the inward channel. The inner layer lines all the out- 
ward channels and the lash chambers, and is of the same nature 
as the outer layer, except in the lash chambers, where, as we have 
seen already, it consist of the remarkable lash cells. The middle 
layer isa sort of jelly with stellate angular bodies in it. The 
skeleton, which we have here, is formed by the cells of the middle 
layer, and it consists of a network of fibres of spongin, which both 
chemically and microscopically resemble silk, both substances 
being regarded as horny matter by chemists. 
The eggs of the Sponge are also found in the middle layer. They 
‘are round cells with a large nucleus and nucleolus. In develop- 
ment these eggs divide first into 2 cells, then 4, then 8, and so on 
until a globular cluster of similar cells is formed called a morula, 
from its resemblance to a mulberry. The outer cells now develope 
cilia or lashes; and in this state the young sponge generally escapes 
into one of the outward channels, and is washed away into the 
