diff --git a/lib/execjs.rb b/lib/execjs.rb
index 27d1c18..2fa145e 100644
--- a/lib/execjs.rb
+++ b/lib/execjs.rb
@@ -3,8 +3,8 @@ module ExecJS
   class RuntimeError    < Error; end
   class ProgramError    < Error; end
 
-  autoload :Runtime,  "execjs/runtime"
-  autoload :Runtimes, "execjs/runtimes"
+  autoload :ExternalRuntime, "execjs/external_runtime"
+  autoload :Runtimes,        "execjs/runtimes"
 
   def self.exec(source)
     runtime.exec(source)
diff --git a/lib/execjs/runtime.rb b/lib/execjs/external_runtime.rb
similarity index 98%
rename from lib/execjs/runtime.rb
rename to lib/execjs/external_runtime.rb
index de0b3f2..50515ac 100644
--- a/lib/execjs/runtime.rb
+++ b/lib/execjs/external_runtime.rb
@@ -2,24 +2,24 @@ require "json"
 require "tempfile"
 
 module ExecJS
-  class Runtime
+  class ExternalRuntime
     def initialize(options)
       @command     = options[:command]
       @runner_path = options[:runner_path]
     end
 
-    def exec(source)
-      compile_to_tempfile(source) do |file|
-        extract_result(exec_runtime(file.path))
-      end
-    end
-
     def eval(source)
       if /\S/ =~ source
         exec("return eval(#{"(#{source})".to_json})")
       end
     end
 
+    def exec(source)
+      compile_to_tempfile(source) do |file|
+        extract_result(exec_runtime(file.path))
+      end
+    end
+
     def available?
       command = @command.split(/\s+/).first
       `which #{command}`
diff --git a/lib/execjs/runtimes.rb b/lib/execjs/runtimes.rb
index 1d56913..dae435c 100644
--- a/lib/execjs/runtimes.rb
+++ b/lib/execjs/runtimes.rb
@@ -9,7 +9,8 @@ module ExecJS
     end
 
     def self.define_runtime(name, options)
-      runtimes.push runtime = Runtime.new(options)
+      klass = options[:as] || ExternalRuntime
+      runtimes.push runtime = klass.new(options)
       const_set(name, runtime)
     end
 
diff --git a/test/test_execjs.rb b/test/test_execjs.rb
index 3ec4610..dfd1ad0 100644
--- a/test/test_execjs.rb
+++ b/test/test_execjs.rb
@@ -11,10 +11,10 @@ class TestExecJS < Test::Unit::TestCase
   end
 
   def test_runtime_available
-    runtime = ExecJS::Runtime.new(:command => "nonexistent")
+    runtime = ExecJS::ExternalRuntime.new(:command => "nonexistent")
     assert_equal false, runtime.available?
 
-    runtime = ExecJS::Runtime.new(:command => "ruby")
+    runtime = ExecJS::ExternalRuntime.new(:command => "ruby")
     assert_equal true, runtime.available?
   end
 end