134 



METAZOAN PHYLA 



animals and man, such as the muscle which shuts Oif the stomach from 

 the intestine, that which guards the exit from the bladder, and that 

 which controls the passage of egested matter from the posterior end of the 

 alimentary canal.) 



158. Reproduction. — Reproduction is both sexual and asexual. The 

 asexual mode of reproduction involves the gradual formation of external 

 buds which arise near the point of attachment of the parent. After grow- 

 ing for a time thus attached a bud may separate and begin an individual 



.'•'0.-;6V-Vffifb; 





jr:0 ci 



B 



Fig. 56. — Gemmules of a fresh-water sponge, Carterius tubisperma Mills. A, fragments 

 of old sponge and gemmules on a piece of wood. X about 3. B, section of a gemmule 

 showing arrangement of spicules in the shell, the foraminal tubule, and the many enclosed 

 cells which were separated from the tissues of the old sponge. X 78. {Drawn by E. F . 

 Powell.) 



existence. If budding continues and the individuals remani together, 

 a colony is produced (Fig. 53 A). 



In addition to external budding, some sponges have the ability 

 to form gemmules, or internal buds. These are groups of cells which 

 gather together in the middle layer and become surrounded by a siliceous 

 shell. They are formed when living conditions become difficult and thus 

 preserve the life of the organism during such periods. 



The gemmules are produced in the autumn, after which the adults die; 

 in the spring the growing cells of the gemmule escape from the shell 



