How can I set a landing page for a structure with no slug?
You can create change the uri pattern for your structured section to something like { parent.uri }/{ slug } and create an additional entry type landing-page. Your landing page will have the slug liver and all entries below that will have the URI liver/{slug} 

You can load different templates based on the entry type and if the type is your landing page, you'll display all children

Another Approach is to use a template path directly and insert the template liver.twig in your templates folder