334 ARTHUR WILLEY. 
observed in Ctenoplana. Chun was at first in doubt as to the 
criterion by which to homologise the planes of Ctenophores 
with those of Bilateralia, as the axes passing through these 
planes in Ctenophora were equipolar (gleichpolig). 
The way by which he finally arrived at the conclusion that 
the tentacle plane corresponded to the sagittal plane was so 
remarkable that I will give a free translation of his descrip- 
tion. 
“ Naturally,” says Chun, “ we must disregard all accidental 
conditions of asymmetry by which one of the axes (Kreuz- 
achsen) becomes inequipolar. For example, one seldom finds 
a Cestus veneris in which the two band-like halves of the 
body are equal in length. . . . Should, however, one of the 
axes prove to be inequipolar in such a way that constantly an 
essential organ-complex failed to develop on the one half of 
the axis, then we should have a transition to bilateral symmetry 
which would enable us to speak of a dorsal and a veutral sur- 
face. . . . How surprised was I to find a larva which pre- 
sented a remarkable axial disturbance in the funnel-plane [2. e. 
the tentacle plane]! I give it the provisional name of Thoe 
paradoxa, as I have not succeeded in associating it with cer- 
tainty with any adult Ctenophore. It possesses, in fact, only 
a single tentacle apparatus and tentacle [Fangfaden]. Only 
in the course of the later development is a second tentacle 
apparatus differentiated at the other pole of the axis, so that 
the original disturbance becomes gradually levelled out.” 
In Ctenoplana the tentacle axis and the stomachal axis 
are equipolar ; but if we consider about which axis the paired 
structures are situated, we are simply forced to acknowledge 
that the plane of the tentacles corresponds to the sagittal 
plane,—in other words, that the tentacle axis of Ctenoplana 
and Ctenophora corresponds to the longitudinal axis of Bi- 
lateralia. 
Lang’s theory of the origin of Polyclades from Ctenophores 
rests in the first instance on the assumption that the pinnate 
tentacles of Ctenophora and Celoplana are homologous with 
the sensory tentacles of Polyclades; and his above-quoted in- 
