ON CTENOPLANA. 335 
terpretation of the axial relations is framed in accordance with 
this assumption. 
In the first place the fact should be emphasised that under 
no circumstances and from no point of view are the tentacles 
of Ctenoplana bilaterally disposed, but they are biradially 
disposed. 
As mentioned above, it cannot be denied that, in the creeping 
attitude, the tentacles of Ctenoplana present to the onlooker 
the appearance of ordinary transversely paired structures, and 
it may seem difficult to imagine an ancestor of bilateral 
animals with an unpaired tentacle in front and an unpaired 
tentacle behind. But the point is that we have not got to 
imagine this, because in the animals with which we are dealing 
there are no such relations as anterior and posterior, right and 
left. 
As regards the particular homology of the pinnate tentacles 
(Greiftentakel) of Ctenophores with the nuchal tentacles of 
Polyclades, so strongly and, it must be added, plausibly upheld 
by Lang, I venture to think that my observations on Cteno- 
plana, especially as to the double character of the aboral 
circlet of sensory tentacles, justifies me in frankly denying its 
accuracy. From their close relation to the central sensory 
apparatus, and the fact that they are paired about the tentacle 
axis, which I regard as equivalent to the longitudinal axis of 
Polyclades, I suggest that it is much more probable, from their 
relations and function, that the paired multiple sensory ten- 
tacles of Ctenoplana and the polar plates of Ctenophora are 
homologous with the sensory nuchal tentacles of Polyclades, 
than that the latter are homologous with the pinnate tentacles 
of Ctenophora, whose chief function is that of seizing objects 
for food. 
Moreover, from their structure and function, their extreme 
retractility within definite sheaths, and their worm.like 
mobility, it would appear that the pinnate tentacles of Cteno- 
plana and Ctenophora belong to a category of structures 
totally different from that of the nuchal tentacles of Poly- 
clades. They belong, namely, to the same category of struc- 
