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The diatoms (Figure 26.5), members of the Chrysophyta, have organic walls impregnated with silica. The architecture of the diatom wall is exceedingly complex; it always consists of two overlapping halves, like the halves of a petri dish. Division is longitudinal, each daughter Cell retaining half of the old wall and synthesizing a new half.
 
The diatoms (Figure 26.5), members of the Chrysophyta, have organic walls impregnated with silica. The architecture of the diatom wall is exceedingly complex; it always consists of two overlapping halves, like the halves of a petri dish. Division is longitudinal, each daughter Cell retaining half of the old wall and synthesizing a new half.
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Although devoid of flagella, some desmids and diatoms can move slowly over solid substrates. The mechanism of desmid locomotion is not known. The locomotion of diatoms is accomplished by a special modification of ameboid movement. In motile diatoms, there is a narrow longitudinal slot in the wall, known as a ''raphe'', through which the protoplast canmake direct contact with the substrate. Movement is brought about by directed cytoplasmic streaming in he canal of the raphe, which pushes the Cell over the substrate.
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Although devoid of flagella, some desmids and diatoms can move slowly over solid substrates. The mechanism of desmid locomotion is not known. The locomotion of diatoms is accomplished by a special modification of ameboid movement. In motile diatoms, there is a narrow longitudinal slot in the wall, known as a ''raphe'', through which the protoplast can make direct contact with the substrate. Movement is brought about by directed cytoplasmic streaming in the canal of the raphe, which pushes the Cell over the substrate.
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Many fossil diatoms are known
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Many fossil diatoms are known, because the siliceous skeleton of the wall (Figure 26.6) is practically indestructible, and as diatoms are one of the major groups of algae in the Oceans, large fossil deposits of diatom walls have accumulated in many areas. These deposits, known as ''diatomaceous Earth'', have industrial uses as abrasives and filtering agents.
    
== The Natural Distribution of Algae ==
 
== The Natural Distribution of Algae ==
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