458 



EVOLUTION OF ANIMALS 



Part V 



Fig. 22.4. Skeleton of the glass sponge, Euplectella, or Venus's flower basket 

 which has a skeleton of silicious spicules interwoven like basketwork. It is at- 

 tached by "glassy" fibers, in deep water. Common near the Philippine Islands. 

 Young shrimps often enter the basket and become permanently imprisoned there. 

 (Courtesy, American Museum of Natural History, New York.) 



ful whether sponges could have attained their relatively large size without these 

 latticed frames. 



The skeletons of calcareous and "glass sponges" are composed of different 

 material, but both are built of units called spicules. Spicules are secreted by 

 special ameboid cells, some of them by one cell, others by two or more to- 

 gether, a cooperation that is rare among sponge cells except in spicule pro- 

 duction. The secretion of a single shaft of spicule (monaxon) is begun within 

 a cell as a minute axial thread around which calcium carbonate is deposited. 



