Code to make fields invisible... (jQuery)

The following form should only be validated when fields are visible.  The other, "hidden" fields should not be validated.  My question is, how do I make fields "hidden?"  When a user selects "section one" from the dropdown menu, I want section one to be shown and all other sections to be hidden.  Likewise, if the user selects "option two," it will be shown and all other sections will be hidden.

<script type="text/javascript">

$().ready(function() {
	
	// validate signup form on keyup and submit
	$("#signupForm").validate({
                  ignore:":hidden",
		rules: {
			firstname: "required",
			lastname: "required",
			username: {
				required: true,
				minlength: 2
			}
		},
		messages: {
			firstname: "Please enter your firstname",
			lastname: "Please enter your lastname",
			username: {
				required: "Please enter a username",
				minlength: "Your username must consist of at least 2 characters"
			},
		}
	});

});

</script>

<form action="" method="post">

<select>
  <option value="volvo">Option one</option>
  <option value="saab">Option two</option>
</select>

<!-- BEGIN OPTION ONE -->

		<p>
			<label for="firstname">Firstname</label>
			<input id="firstname" name="firstname" />
		</p>
		<p>
			<label for="lastname">Lastname</label>
			<input id="lastname" name="lastname" />
		</p>

<!-- END OPTION ONE -->

<!-- BEGIN OPTION TWO -->

		<p>
			<label for="firstname">Firstname</label>
			<input id="firstname" name="firstname" />
		</p>
		<p>
			<label for="lastname">Lastname</label>
			<input id="lastname" name="lastname" />
		</p>

<!-- END OPTION TWO -->

</form>