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. 



