SO select list items

HTML

<div id="itemsListBox">
    <ul>
        <li>ONE</li>
        <li class="selected">THREE</li>
        <li>FOUR</li>
        <li>FIVE</li>
        <li>SIX</li>
        <li class="selected">SEVEN</li>
        <li>EIGHT</li>
        <li>NINE</li>
    </ul>
</div>
<button id="downArrow">Down</button>

CSS

.selected {
    background: green;
}

JavaScript

$("#downArrow").click(function() {
    $(".selected").after().addClass("selected");
});