SPONGIDA. 
245 
They develop, however, regularly from the egg, and the 
cells acquire their independence only at a late date in de¬ 
velopment. Some of the cells bear cilia, or flagella, and 
drive the water through numerous channels into the cen¬ 
tral cavity, whence it is discharged by one opening. Each 
cell of the Sponge feeds itself from the particles contained 
in the water circulating through the channels. 
The Sponge-individual contains one exhalant orifice 
(osculum), with the channels leading into it. An ordinary 
Fig. 190.—Skeleton of a Horny Sponge. 
bathing-sponge constitutes a colony of such individuals, 
which are not definitely marked off from each other. 
Other Sponges have only one osculum, and such are a 
single .individual. 
Some few Sponges have no skeleton. Most have one 
of horny fibres, strengthened with siliceous spicules. These 
last are absent in the commercial Sponges, and in them 
the horny fibres are much tougher than in most Sponges. 
