84 



INVERTEBRATE ZOOLOGY 



Reproduction. — Asexual reproduction through budding is 

 common in sponges. By this means masses of individuals are 

 produced thereby forming colonies. These colonies may be 

 either groups of distinctly separable individuals with only a 

 common region near the base, or they may be so intimately 

 fused as to render the recognition of individuals impossible. 

 Sponges possess great power of regeneration. In the commercial 

 sponge fisheries, advantage is taken of this fact and small frag- 

 ments are planted and allowed to grow before harvesting. 



Fig. 50 A. — Embryo of Grantia. Blastula stage within embryonic chamber 

 of parent individual. {After Dendy). 



Fig. 50 B.- — Proterospongia, showing collared flagellate cells. {Redrawn from 



Kent) . 



Gemmule formation is another type of reproduction found in 

 fresh-water sponges. Groups of cells called gemmules or internal 

 buds become separated from the surrounding tissues by confining 

 membranes. These gemmules are capable of withstanding 

 desiccation and other adverse circumstances which are fatal 

 to the living sponge and are therefore of great importance in 

 the life cycle of sponges. 



Sponges are usually hermaphroditic, but since in any individual 

 the germ cells of the male usually mature before those of the 

 female they are said to be protandrous. Eggs are fertilized 

 within the parent and there undergo segmentation (Fig. 50 A). 

 The formation of the germ layers and the transformation of the 

 larva present conditions wholly unlike these found typically 

 in the Metazoa. The blastomeres resulting from cleavage of 



