46 ART. 8. 1. IKEDA : THREE NEW AND 



fibers {m.f.) constitute the wall of the vessel. The inner epithel- 

 ium may with propriety be called the endothelium, while the 

 outer is the continuation of the peritoneal lining of the perihœ- 

 mal cavity. This structure of the vessel wall is clear in the 

 present species, though it does not agree quite with our previous 

 knowledge derived from other Echiuroids. Thus, Rietsch ('86) 

 says that in Thalassema neptuni and Echiurus j^ttHcisii the vessel 

 wall is composed of only the peritoneal layer. Spengel ('8o) 

 states for the same of Echiurus pallasii : " Ich weiss nicht, ob 

 eine innere Zellenauskleidung vorhanden ist. Ich fand nur eine 

 dünne Membran mit eingestreuten Kernen und in oder auf dieser 

 liegend Muskelfasern, vorwiegend longitudinale Bündel bildend, 



und als äusserste Schicht einen Peritonealzellenbelag " 



[p. ölO). One sees at once that the " dünne Membran mit 

 eingestreuten Kernen " in the above extract corresponds to the 

 endothelial layer. 



The median vessel of the proboscis is of essentially tlie 

 same structure as is known from Bonellia 7uinor, Thalassema 

 neptuni and Echiurus pallasii (Rietsch, '86), except in some 

 not unimportant points. The reader is referred to fig. 47, which 

 represents a part of a cross-section through the vessel in ques- 

 tion. On the external side of the vessel wall and lying against 

 the mesenchyme tissues {c.t.c.y m.f.b.), is a, row of tall club-like 

 cells arranged in an epithelium-like manner {ejd.). Tliey have 

 the swollen and rounded end directed outwards, and sit with the 

 opposite truncated end on a thin layer of circular muscle fibers. 

 (c.vi.). Some of the mesenchyme cells in the immediate sur- 

 rounding are seen to insert their processes between the cells of 

 the row. Inside the circular nuiscle fibers is a layer, which on 

 cross-sections might at first sight be mistaken for an epithelium 



