II STRUCTURE HEART AND BLOOD 49 



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, 7i.ni) 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 Ascidia (Fig. 23) the heart is an elongated fusiform tube 

 placed 011 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 \vhich 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 



