21 



rectum, close to the atrial aperture, is ver^^ thin- walled, 

 with a slight thickened edge at the anus, but no sphincter. 



Heaet 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 PI. 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 

 (PI. IV., fig. 9,p.c.), which is a part of the original coelom. 



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 PI. IV., fig. 10), shutting in a portion 

 of the external space (the blastocoele of the embryo) and 

 having open ends which connnunicate 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 (nmscular) 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 lacunoe are probably derived, like the cavity of 

 the heart, from the blastocoele of the embryo, and are not 

 (like the pericardium) a derivative of the coelom. 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 



