II - Studies in Tunicate development. Part V - The evolution and classification of Ascidians

Abstract

The purpose of the present paper is twofold. The development and variability of the heart, pericardium, and epicardium throughout the group will be described, while an attempt is made to trace the probable course of evolution within the ascidians and to base upon it a revised classification as it concerns the major divisions of the class. The mode of origin of the heart and pericardium has long been recognized as one of the strong links between the tunicates and vertebrates, while the epicardium is a highly significant structure assumed by many to be concerned primarily with budding. Current classifications of ascidians, such as those of Lahille and Seeliger, are based upon the nature of the branchial sac, an insecure foundation since branchial structure is to a great extent an expression of the size of the organism ; so that a more general basis is desirable.