I have added the Prototype library to my site, then add the following code. but when i click the span, the ul content is not hidden. the link href still work.

Code:
Event.observe(window, 'load', function() {
    Event.observe('.block-category li.parent span', 'click', function(e){
        $('.block-category li.parent ul').toggle();
        e.preventDefault();

    });
});
html:

Code:
<div class="block block-category">
<li class="level-top  parent">
<a href="example.com/...."><span>text one</span></a>
<ul>
<li><a><span>....</span></a></li>
<li><a><span>....</span></a></li>
<li><a><span>....</span></a></li>
<li><a><span>....</span></a></li>
</ul>
</li>

<li class="level-top"><a href="..."><span>....</span></a></li>

</div>
thank you