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 gain 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 con¬ 
tained in the water. 
The Sponge-individual consists of one exhalant orifice, 
with the channels leading into it. An ordinary bathing- 
Fig. 190.—Horny Skeleton of a Sponge. 
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. 
