In this paper the author maintains the proposition, that the class Tunicata may be conveniently divided into two sub-classes, viz., the Fixed or Stationary, and the Free or Locomotive, of at least nearly equal value in a zoological point of view, in opposition to the opinion commonly entertained, that the so-called Pelagic Tunicata compose a group only commensurate with the groups of the Compound, the Social and the Simple, into which the Fixed Tunicata have been divided by Milne-Edwards and others.