1^0 THE ANIMAL KINGDOM 



hooks that serve to anchor them to the bottom. When the environment 

 is suitable (in the spring) the gemmules sprout into young sponges. 



Sponges are simple animals, poorly coordinated, and it is not sur- 

 prising that they can easily regenerate lost parts. Indeed, if the more 

 complex sponges with several oscula are cut in half, there are no lost 

 parts. The ability of sponge cells to reorganize was demonstrated by 

 E. V. W^ilson in 1907. Sponges squeezed into a dish through fine silk 

 cloth are disaggregated into minute cell clumps. The choanocytes swim 

 about on the bottom by their flagella, and the amebocytes crawl. When- 

 ever cells come in contact, they remain together. The bottom of the 

 dish is soon covered with balls of cells, each of which develops into a 

 tiny sponge if it includes both choanocytes and amebocytes. If the mass 

 is very small, the choanocytes congregate on the outside and the organism 

 resembles a colonial choanoflagellate. If the mass is large enough the 

 choanocytes form chambers covered by the amebocytes. 



Questions 



1. Diagram the three structural types of sponges. 



2. Which sponges are found in fresh water? 



3. How do the Demospongia differ from the Calcarea? 



4. Discuss gastrulation in the sponges. 



Supplementary Reading 



A general and thorough account of the phylum is given in The Invertebrates, volume 

 I, by L. Hyman. 



