A GUIDE TO THE SPONGES 



231 



grow together and finally form a perfectly rigid network. The 

 microscleres on the other hand are not supporting in function. 

 In fact in most cases their use is imknown. They are foiind 

 embedded in the fleshy parts of the sponge and are so minute 

 as to be distinctly visible only under a high power of the mi- 

 croscope. They are extremely valuable in determining species. 



REPRODUCTION AND DEVELOPMENT. 



Sponges may reproduce either by budding {asexual repro- 

 duction) , or by means of eggs (sexual reproduction) . Reproduction 

 by budding is brought about by an outgrowth of cells from the 

 side of the sponge involving all three layers. This finally develops 



FIG. 15.— FRESH WATER SPONGES {S/on^i/Za sp.) 



into a miniature of the parent sponge, as far as structure is con- 

 cerned, becomes narrowed at the base until it is only attached 

 by a stem, and finally drops off. It then becomes fixed to the 

 sea-bottom and grows to maturity. Sexual reproduction on the 

 other hand is only effected by the union of sexual elements 

 within the tissues of the parent sponge. The male and female 

 reproductive cells originate in the mesoderm of the same in- 

 dividual and iinite to form the fertilized unicellular egg. The 

 larva is developed from the one-celled stage, by a process of cell 

 division or cleavage. It passes through 2-, 4-, and 8-celled 



[17] 



