ASCIDIA. 57 
rectum, close to the atrial aperture, is very thin-walled, 
with a slight thickened edge at the anus, but no sphincter. 
HEART AND CIRCULATION. 
The soluble product of the food which has been digested 
passes through the wall of the alimentary canal, and 
enters the numerous small blood spaces in the connective 
tissue on both sides of the stomach and intestine. These 
lead, by the cardio-visceral vessels, to the dorsal end of 
the heart (see Pl. III., fig. 10), which is merely a delicate 
tube, irregularly swollen in the middle, placed behind the 
stomach, and projecting into a space, the pericardium 
(P1. 1V., fig. 9, p.c.), which is a part of the original ccelom. 
The wall of the heart is continuous along one edge 
(that next the stomach) with that of the pericardium, and 
the heart is to be regarded as a tubular invagination of the 
pericardial wall (see Pl. IV., fig. 10), shutting in a portion 
of the external space (the blastoccele of the embryo) and 
haying open ends which communicate with the large blood 
sinuses leading to the branchial sac, to the viscera, and to 
the body-wall and test. The cavity of the heart is not 
divided and has no valves. Its wall is formed of a single 
layer of epithelio-muscular cells, the inner (muscular) ends 
of which are cross-striated fibres running round the heart 
—the only striated muscle found in the body of the 
Ascidian. The larger channels through which the blood 
flows are lined by a delicate endothelium, the smaller are 
merely spaces in the connective tissue. All the blood 
spaces and lacunz are probably derived, like the cavity of 
the heart, from the blastoccele of the embryo, and are not 
(like the pericardium) a derivative of the ceelom. The 
wall of the pericardium is simple squamous epithelium. 
From the ventral end of the heart the blood is conveyed 
by the branchio-cardiac vessel and the great ventral vessel 
