Respiratory Gas Exchange in Ascidans: An Almost Diffusion Limited Animal with a Cardiovascular System

Abstract

The ascidians are a class of the sub Phylum Tunicata, and part of the Phylum Chordata. Characteristically they are sessile marine organisms which have a motile larval stage. They are believed to represent an ancient stock from which other chordates and eventually the vertebrates have evolved. The geometry of the motile (larval) stage and of the ancestral animal is simple in the extreme, supporting the view that this species represents a very early and fundamental form in the evolution of animals. Berrill (1955) has discussed this aspect of the ascidians and has pointed out the evolutionary increase in body size which has occured, from an original animal of less than one mm length, to present day organisms of 10 cm or more. The appearance of a tunic being seen as an adaptation to provide mechanical support and protection for this increased body. The archaic form was a typical diffusion limited animal, whereas it is by no means so obvious that this is so for modern much larger organisms, which possess a cardiovascular system, although rather an unusual one in which the heart regularly and rythmically reverses the direction of flow of blood.