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>
    <allpages gapcontinue="Zona_límbica" />
  </query-continue>
  <query>
    <pages>
      <page pageid="136" ns="0" title="The Evolutionary Bases for Sustainable Behavior: Implications for Marketing, Policy, and Social Entrepreneurship (Griskevicius et al. 2012)" contentmodel="wikitext" pagelanguage="es" touched="2014-09-22T18:21:22Z" lastrevid="1266" counter="59285" length="41189" />
      <page pageid="144" ns="0" title="The Omnivore&#039;s Dilemma: The Evolution of the Brain and the Determinants of Food Choice (Armelagos, 2010)" contentmodel="wikitext" pagelanguage="es" touched="2014-09-22T18:02:14Z" lastrevid="1264" counter="19683" length="30469" />
      <page pageid="16" ns="0" title="Tipografía de estado estable (SST)" contentmodel="wikitext" pagelanguage="es" touched="2014-05-09T17:25:37Z" lastrevid="890" counter="10078" length="2186" />
      <page pageid="57" ns="0" title="Tu cerebro lo es todo" contentmodel="wikitext" pagelanguage="es" touched="2014-05-09T18:18:12Z" lastrevid="914" counter="6073" length="1224" />
    </pages>
  </query>
</api>