2,00 HIROSHI OHSHIMA 
shaped thickening on its edge, which he believed to be homo- 
logus with the ciliary band of the auricularia. I was unable 
to verify Ludwig’s opinion, but the stage which I. call 
dipleurula is homologous with the auricularia in respect to the 
arrangement of the internal vesicles. 
The arrangement of the ciliary bands as well as the degrees 
of development of the alimentary tract cannot, I believe, help 
us in discussing homologies among different forms, because 
they vary in degree according to different modes of livmg. In 
the free-living auricularia of Labidoplax digitata the 
alimentary canal is well differentiated into fore-, mid-, and 
hind-gut, and the ciliary band is typically developed, as is well 
known from the records of many observers. ‘The embryo of 
Synaptula hydriformis developing inside the mother’s 
body-cavity retains an elliptical shape of body, showing no trace 
of any ciliary band, the gut beimg quite rudimentary (Clark, 
7, p. 62). Another viviparous form, Chiridota rotifera, 
shows an intermediate feature between the above two (Clark, 
9, p. 501). 
The division of the twisted archenteron in the old gastrula 
occurs first at the postero-ventral end of the second part, where 
a solid cell-mass with obliterated lumen connects the divided 
portions for a while (PI. 8, fig. 13). The larger vesicular portion, 
consisting of the first and second parts of the archenteron is 
now to be called hydro-enterocoele or vaso-peritoneal vesicle, 
while the smaller one which was the third part is the future 
cut. The latter has a very narrow lumen, in most cases being 
still continuous with the exterior through the blastopore. 
The next change occurring in the larva is the displacement, 
change of shape, and division of the hydro-enterocoele, and the 
enormous multiplication of the mesenchyme cells which fill up 
the blastocoele, so that no external examination of the internal 
structure on clarified material is now possible. 
The anterior part of the hydro-enterocoele, which in the late 
gastrula was concave on the right side (PI. 8, fig. 14 4, ary), 
now moves round to the right across the dorsal side and becomes 
narrow in breadth (Pl. 8, fig. 16.4, hy). The posterior part 
