230 SELIG HECHT 
Ascidia atra lives attached to rocks, in most cases well under. 
low water. The attachment of the larger animals is by the 
posterior edge; in the smaller individuals a portion of the left 
side is frequently attached as well. The species is well distributed 
throughout the Bermuda Islands. Although animals for this 
research were obtained from many regions, the main supply 
came from localities very near Agar’s Island. 
Ascidians first became of interest during the last half of the 
nineteenth century. Their significance in relation to the origin 
of vertebrates, which was first made apparent by the work of 
Kowalevsky (’67), resulted in innumerable researches on the 
anatomy and embryology of the group. Since that burst of 
activity, sixty years ago, the knowledge of ascidians has not 
kept pace with the newer points-of-view. As a consequence, 
little, indeed, is known of the life and activities of these animals. 
It is with the hope of supplying this deficiency that the present 
series of papers is presented. 
The work was done at the Bermuda Biological Station with 
the assistance of a grant from the Humboldt Fund. I wish to 
acknowledge the kindness of Prof. E. L. Mark, who made this 
grant possible, and at whose invitation the experiments were 
performed in Bermuda. To the Resident Naturalist, Dr. W. J. 
Crozier, the depth of whose friendship was manifest in many 
ways, I extend my frank admiration and thanks. 
My chief indebtedness, however, is to Prof. G. H. Parker. 
His teachings and researches, which have the “rare merit of 
combining both anatomical and physiological view-points,” 
have influenced my work and thought. It is a privilege to 
express my gratitude for the inspiration which he has given me. 
II. EXTERNAL APPEARANCE 
1. Orientation 
At the outset of this account of the physiology of Ascidia 
atra, it is necessary to define the various surfaces and planes of 
the body. The earlier writers on ascidian anatomy were far 
from agreed on the application of such terms as anterior, poste- 
