You are looking at the HTML representation of the XML format.
HTML is good for debugging, but is unsuitable for application use.
Specify the format parameter to change the output format.
To see the non HTML representation of the XML format, set format=xml.
See the complete documentation, or API help for more information. 
<?xml version="1.0"?>
<api>
  <query-continue>
    <allfileusages gafcontinue="Base_structure_wiki_box_5.PNG" />
  </query-continue>
  <query>
    <pages>
      <page ns="6" title="File:Base Structure WIKI.Gertjan.png" missing="" />
      <page pageid="551" ns="6" title="File:BQE benthic invertebrates.jpg" />
      <page pageid="552" ns="6" title="File:BQE fish.jpg" />
      <page pageid="550" ns="6" title="File:BQE macrophytes.jpg" />
      <page pageid="549" ns="6" title="File:BQE phytoplankton.jpg" />
      <page pageid="462" ns="6" title="File:Bachgut.jpg" />
      <page pageid="463" ns="6" title="File:Bachschlecht.jpg" />
      <page pageid="821" ns="6" title="File:BankRemoval.jpg" />
      <page pageid="6595" ns="6" title="File:Base structure wiki box 1.PNG" />
      <page pageid="6596" ns="6" title="File:Base structure wiki box 2.PNG" />
    </pages>
  </query>
</api>