912 HIROSHI OHSHIMA 
in the five primary tentacles (compare Becher, 8, p. 4). 
It is an open question how the mid-ventral radial canal behaves 
in the early stages of Sphaerothuria bitentaculata, 
which possesses only eight tentacles when adult, the mid- 
ventral radial canal supplying no tentacles at all. 
Polian Vesicle.—the Polian vesicle appears at the free 
end of the ventral limb of the open hydrocoele ring, directed 
posteriorly and lying inside the enterocoele vesicle (Pl. 9, 
figs. 23.4,B; 24D; pv). It may often appear after the closure 
of the ring. 
Stone-Canal.—About at the same time as the appearance 
of the Polian vesicle the pore-canal swells out dorsally at its 
middle part, and as a result of it the canal slightly bends at this 
point (Pl. 9, fig. 25, as). In transverse section this swelled 
part shows a very characteristic feature in that the wall of the 
axial side is‘of a very high epithelium, while along the dorsal 
side the wall is very thn. Bury (5, 1889, p. 427; Pl. xxxix, 
fig. 26) first noticed this structure in a Cucumaria, and 
considered it as a vestigial anterior enterocoele, the presence 
of which he had proved in auricularia. Ludwig (22, p. 609) 
observed it in the fifth-day larva of C. planei and ealled 
it the madreporie vesicle, with an assumption that it is only 
a secondary outgrowth. In his later paper Bury (6, 1895, 
pp. 538-4) insisted upon his former view, and suggested that 
future and closer examinations would reveal changes similar 
to those in auricularia, proving its origin from the enterocoele. 
According to Newth (86, p. 687) this enlargement occurs 
by an up-pushing of the antero-dorsal wall of the canal on the 
third day in C. saxicola andC. normani. On the next 
day the cells of the antero-dorsal wall of this vesicle become 
large and clear. This swelling up of the cells seemed to him 
to be a preliminary stage in the thinning out of the part as seen 
by Bury, Ludwig, &e. I was unable to find either the 
change which Bury suggested to be present or the swelling 
up of the cells in the early stages of this structure. The same 
structure has further been proved to be present in Phyllo- 
phorus urna by Ludwig (24 p. 98) and by Russo 
