12 ART. 5. — T. AIDA. 



eral aspect of the alimentary canal. Ovary unpaired 

 and flat; testis paired when young, but united to a 

 single mass in older stages. Tail and its musculature 

 broad. No subchordal cells j^resent. 



The wall of the trunk is thin, and the branchial cavity and the 

 mouth are spaceous so that the trunk has a cylindrical shape. 



The oikoplast epithelium covers only the branchial portion 

 of the trunk ; the œsophagus, the stomach and the intestine lying 

 entirely behind it. In the lateral oikoplast zone there is a group 

 of remarkably elongate, obliquely arranged cells, and also another 

 group of large cells directly above these (fig. 13). The endostyle 

 is slender and long ; its posterior end lies close to the anal 

 opening. 



The atrial aperture of this species shows a peculiar feature. 

 There is no atrial canal, and the branchial cavity communicates 

 directly with the external world by a single aperture. This 

 aperture has on the anterior side a small semicircular notch, the 

 margin of which is thickend by the presence of ciliated cells 

 (fig. 18). As this ciliated notch is similar in structure to the 

 spiracle or the inner opening of the atrial canal of other species, 

 the single aperture may be considered to have been formed 

 by obliteration of the atrial canal and by union of the sj)iracle 

 and the outer opening of the atrial canal, the ciliated notch re- 

 presenting the renniant of the spiracle. 



The oesophagus is strongly curved and forms a semicircle 

 (figs. 13 and 16). The left lobe of the stomach is flat and 

 rectangular. It has a short finger-like blind-sac at the upper 

 posterior corner, which does not lie close along the œsophagus 

 but is widely separated from it (hi., fig. 13). A row of large 



