Ul STRUCTURE—HEART AND BLOOD AGS 
sac wall into rectangular areas called “ meshes.” One such mesh, 
containing eight stigmata in a row, is seen in Fig. 22, A. The 
internal longitudinal bars bear papillae at the angles of the 
meshes, and occasionally in intermediate positions. There are 
frequently horizontal membranes (Fig. 22, B, 4.m) attached to 
the transverse vessels between the papillae. There are many 
“connectives ” running from the outer wall of the branchial sac 
to the mantle outside, and allowing the blood in the transverse 
vessels to communicate with that in the sinuses of the mantle 
(see Fig. 19, con). : 
Heart and Circulation.—It is one of the notable features of 
the Tunicata that the circulation is not constant in direction, 
but is periodically reversed. 
The blood of Ascidians is in the main transparent, but usually 
contains certain pigmented corpuscles in addition to many 
ordinary leucocytes or colourless amoeboid cells. The pigment in 
the coloured cells may be red, yellow, brown, or in some cases 
blue or opaque white. The blood may reach the branchial sac 
either from the dorsal or from the ventral median sinus according 
to the direction in which the heart is beating at the moment (see 
below); and it is a most interesting and beautiful sight to see 
the circulation of the variously coloured corpuscles through the 
transparent vessels, and the lashing of the cilia along the edges 
of the neighbouring stigmata in a small Ascidian under the 
microscope. 
In Aseidia (Fig. 23) the heart is an elongated fusiform tube 
placed on the ventral and posterior edge of the stomach, project- 
ing into a space (the pericardium) which is a part of the original 
coelom, the remainder of which is represented in the adult by 
the reproductive and renal cavities. The wall of the heart is 
continuous along one edge with that of the pericardium, and the 
heart is to be regarded as a tubular invagination of the pericardial 
wall, shutting in a portion of the surrounding space (the blastocoel 
of the embryo), and having 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 there are 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 muscular tissue found in the body. Waves of contrac- 
VOL. VII E 
