ONE-CELLED ANIMALS AND SPONGES. 
125 
these cavities are provided with flagella which by their 
constant movement keep the water moving along from 
the inhalent to the exhalent openings. As the water 
passes along it brings 
within reach of t h e 
flagella minute animals 
and plants which are 
seized and pushed back 
into the cells where 
they are dissolved and 
assimilated in much 
the same way as the 
food of the Amoeba is 
assimilated in its cell. 
The breathing, too, is 
carried on by the indi- Fig - 109.— Sponge Spicules, 
vidual cells as in the 
Amoeba. Nutriment from the endoderm cells is passed 
along from cell to cell to nourish the rest of the body. 
From this nutriment the material for building the 
skeleton is secreted. This skeleton is often in the 
shape of spicules of hard material. Spicules may be 
calcareous, silicious , or horny , producing these three 
kinds of sponges. Only the horny or keratose sponges 
are of any commerical use. 
So nearly independent are the individual cells com¬ 
posing the sponge-structure that if a few of them be 
separated from the original body, they go on living 
and divide and subdivide, making new cells and build¬ 
ing the structure of a new sponge. If a living sponge 
be cut into hundreds of pieces, each piece grows into 
an independent animal. 
Such an aggregate of cells may be considered as 
only a transition step between a Protozoon and a 
many-celled animal of more highly specialized struc¬ 
ture. Hence some naturalists have classed sponges as 
Protozoa, some make a separate sub-kingdom Porifera, 
while others class them with the sub-kingdom Ccelen- 
