24 Dr. Asajiro Oka : 



stomach (PL III., fig. 19). The tubules are round or oval in cross 

 section and measure 0,02-0,025 mm. in diameter. They are placed 

 on the outside of the intestinal epithelium and are generally in close 

 contact with it (PL III., fig. 21). The course of these tubules is 

 wavy throughout. For the greater part they run parallel with the 

 axis of the intestine. In the part lying nearest to the stomach, 

 however, they are found to form two or three rings encircling the 

 intestine. The duct is lined with cubical cells (PL III., fig. 19), 

 but the tubules themselves are composed of much flatter cells. 



Blood vascular system. The heart lies in the loop formed by 

 the alimentary canal between the stomach and the ascending 

 portion of the intestine. It is a fusiform tube with undulating 

 walls, and is enclosed in an outer delicate membranous sac, the 

 pericardium, which fills the greater part of the space between the 

 stomach and the intestine (PL III., fig. 20). The heart is attached 

 to the pericardium along a line on that side which lies next the 

 intestine. The wall of the heart is not of the same thickness 

 throughout, but is thinnest along the line where it joins the peri- 

 cardium and becomes gradually thicker as it is traced to the other 

 side, where it is sometimes as thick as the wall of the intestine. 

 (Pl. III., fig. 25). A number of muscle fibres are clearly discernible 

 in its tissue. In some sections it can be distinctly made out that 

 the walls of the heart and pericardium are directly continuous 

 witli each other, and that the heart is simply an invagination of 

 the pericardium. 



The heart is open at both ends. The blood flowing out of 

 the heart passes into definite canals channeled out in the connec- 

 tive tissue filling the space between the body wall and the internal 

 organs (PI. II., fig. 14; PI. III., figs. 18 and 25, hi sp.). The blood 

 corpuscles are round or oval cells with distinct nuclei (PL III., fig. 

 23). They are gathered in some of the blood spaces while they 

 are wholly absent in others. 



As stated before, each zooid sends out from the posterior end 

 of abdomen an ectodermal appendage. This is a thin walled tube 

 of varying width divided longitudinally into two cavities by a 

 delicate septum, so that in reality it is formed of two vessels run- 



