140 ANIMAL BIOLOGY 



In others it may be formed entirely or in part of a horny 

 substance, as in our common bath sponges. 



Sponges may build up large masses by budding; in this 

 way they frequently give rise to detached individuals; 

 but they also reproduce sexually by means of ova and 

 spermatozoa. The eggs of many species produce free- 

 swimming larvae which finally settle down and develop 

 into a small attached sponge. While most sponges are 

 marine, there are a few fresh-water forms (Spongilla and 

 allied genera). The sponges of commerce are fished up 

 in certain localities, by long rakes, or by means of divers. 

 The animal matter is allowed to decay, and the horny 

 residue is bleached and cleaned before the sponge is ready 

 for use. 



