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EVOLUTION OF ANIMALS 



Part V 



Fig. 21.14. Shells of marine foraminiferans. The majority of them have several 

 chambers. The animal secretes the shell of one chamber but as it grows it slips out 

 of this and secretes a larger chamber and then another until it reaches full size. The 

 whole collection shown here is about seven inches wide. Layers of foraminiferan 

 shells fallen from the surface waters are said to cover two-thirds of the floors of 

 the oceans. (Courtesy, South Kensington Natural History Museum, London.) 



extended from beneath the mushroom-shaped shell-like tests. They are actually 

 shelled amebas and as easily cultured (Fig. 21.13). Difflugias, also ameba- 

 like, are covered by tests inset with minute sand grains. During the asexual 

 division one part of the animal protrudes from the test and its ectoplasm 

 secretes a sticky fluid to which sand grains adhere. After division, the difflugias 

 separate, one covered by the old test, one by the new. Heliozoans or sun 

 animalcules whose bodies are decked with crystal clear filaments are the 

 splendid relatives of amebas. 



Sea Water. Foraminiferans (hole-bearing) are ameboid protozoans that 

 secrete many-chambered shells, most of which are chalky; others are of 

 chitin or silica (Fig. 21.13). The young foraminiferan makes a shell with an 

 opening from which it extends its body as a snail does. As it grows, its proto- 



