NOTE ON THE CILIATED PIT OP ASCID1ANS. 141 



and stops just behind the atrial pore. The whole of the ali- 

 mentary canal behind the pharynx is situated on the left side 

 of the partition (fig. 12, St. and Int.). Thus the alimentary 

 canal, embedded in its own portion of the " pseudocoele," is 

 completely surrounded by the atrial cavity except at those 

 points where strands pass off connecting its " pseudocoele " 

 with the " pseudocoele" in the body wall outside the atrial 

 cavity, and forming the mesenteries described above. 



The atrial cavity is lined throughout by a layer of fiattish 

 cells, lying upon a basement membrane (figs. 12 and 15, 

 A. Ep.). The cells are rounded at their free ends, and have 

 large nuclei. This layer, of course, extends over the portions 

 of the pseudocoele surrounding the alimentary canal, and over 

 the mesenteries. 



The atrial pore is placed not far from the mouth on a 

 short papilla. 



Alimentary Canal. 



The mouth leads into the buccal cavity, which passes into 

 the large pharynx (fig. 4, Ph.). The endostyle extends round 

 the posterior end of the pharynx, ceasing at the point where 

 the oesophagus is given off. The oesophagus (fig. 4, Oes.) is 

 short, and soon opens into the large sack-like stomach (fig. 4, 

 St.), which, on its external surface, is marked by deep longi- 

 tudinal ridges. The stomach lies on the left side of the 

 pharynx, its long axis being at right angles to that of the 

 latter. It passes into the intestine (fig. 4, Int.), which almost 

 at once bends upon itself and runs back across the pharynx, 

 almost parallel with the stomach. On reaching the level of 

 the dorsal side of the pharynx it turns forwards almost at a 

 right angle, and runs straight to the anus (fig. 4, A.), which is 

 situated at a short distance from the atrial pore, at the point 

 where the partition dividing the atrial cavity ceases. 



Shortly after leaving the stomach the intestine appears to 

 be dilated for a short distance (fig. 4, Int. L.) ; this appearance 

 is due to its being surrounded here by a layer of liver-cells. 



The buccal cavity is lined just within the mouth by a 



