SPONGIDA. 



245 



Tliey develop, liowever, regularly from the egg, and tlie' 

 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 cavitj', whence it is discharged b^"^ one opening. Each 

 cell of the Sponge feeds itself from the particles contained 

 in tiie 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. 



