332 ARTHUR WILLEY. 
some details. The relation to the otolith-bearing portion of 
the sense-organ is identical in both cases. The polar plates of 
Ctenophores are ciliated, as are the sensory tentacles of Cte- 
noplana. Moreover in the Beroide, according to Chun, the 
thickened margin of the polar plates does not form a simple 
ridge, but is raised up into a series of lappets. This is a very 
remarkable correspondence, and after my observation of the 
double, paired character of the sensory tentacles of Cteno- 
plana I think there can be no doubt that the latter are homo- 
logous with the polar plates of Ctenophores. 
The comparison of the gonads of Ctenoplana with those of 
other forms is not such a simple matter. They agree with 
those of the Ctenophora in being developed about the walls of 
diverticula of the gastro-vascular system, and with those of the 
Potyclades in being enveloped in a tunica propria. But they 
differ from both in the possession of ducts opening directly to 
the exterior. In the Ctenophora the genital products fall into 
the meridional vessels, and are discharged through the mouth ; 
while in the Polyclades, according to Lang, the tunice pro- 
prize which envelop the innumerable testes open into a system 
of intra-cellular genital capillaries which eventually convey the 
sexual products to the vas deferens on each side, by which 
they are ultimately led to the ventrally placed external genital 
pore. 
We now come to the critical consideration of the axial rela- 
tions of Ctenoplana. The problem to be solved is the follow- 
ing :—To what do the planes of the tentacles and of the - 
stomach respectively correspond in bilateral animals? Does 
the tentacle plane of Ctenoplana (‘Trichterebene of Ctenophores) 
correspond to the sagittal plane of bilateral animals or to the 
transverse plane ? 
We shall find, if it has not already appeared evident in the 
foregoing pages, that Ctenoplana unequivocally proves, as I 
think, that the tentacle plane or funnel-plane of it and the 
Ctenophores corresponds to the sagittal plane of bilateral 
animals, and not to the transverse plane. 
At present there exist two interpretations of the axial rela- 
