PRIMITIVE MULTICELLULAR ANIMALS 



external cells move to the interior and produce the choanocytes which line 

 the spongocoel or the flagellated chambers (Fig. 9.13). In simpler forms, the 

 olynthus stage is passed through; in more complex sponges, development may 

 be more direct. The final stages may include much budding and growth, 

 often leading to the formation of a large mass. This budding is comparable 

 with the asexual reproduction occurring in other multicellular animals, al- 

 though it is often difficult to distinguish from the general process of growth. 

 The fresh-water sponges, and some marine forms, produce internal buds, 

 termed gemmules, which are covered with resistant membranes and can sur- 

 vive severe conditions such as freezing and drying. In various other sponges, 

 under adverse conditions, so-called reduction bodies are formed. These con- 

 sist of masses of amoebocytes surrounded by dermal cells. They are less 

 resistant than gemmules, but they serve a similar purpose in preserving the 

 species through periods during which normal life would be impossible. With 

 the return of favorable conditions, the gemmules or the reduction bodies can 

 produce fully developed individuals. 



Regeneration and Reassociation. Regeneration, by which lost parts are 

 restored and even whole individuals are formed from small pieces, is com- 

 monly associated with conspicuous powers of asexual reproduction. The ex- 

 tensive budding and vegetative growth of which sponges are capable would 

 lead us to expect greater powers of regeneration than seem to exist in these 

 animals. Some sponges will regenerate from cuttings, and artificial propaga- 

 tion of commercially valuable sponges has been attempted, without marked 

 success, by planting small cuttings in favorable locations. 



In at least a few types of sponges there is a remarkable capacity of re- 

 association of cells after the organization of the body has been completely 

 disrupted. For example, pieces of Microcwna prolifera, the common red 



Fig. 9.13. Flagellated amphi- 

 blastula larva of a calcareous 

 sponge, Grantia compressa. The 

 developing sponge reaches this 

 stage before it is released from 

 the parent. After a brief free- 

 swimming period, it settles to the 

 substratum and transforms into 

 the sessile adult form. (Redrawn 

 from O. Duboscq and O. Tuzet, 

 1937, Archives de zoologie expen- 

 menlale el generate, vol. 79.) 



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