338 ARTHUR WILLEY. 
On the other hand, a biradial form, like Ctenoplana, 
possesses the potentiality of assuming a strictly littoral 
life, in which the ventral surface is the permanent locomotor 
surface, such an existence leading to a condition of bilateral 
symmetry, according to well-understood physiological prin- 
ciples. 
The ctenophoral plates must have put in their appearance 
for the first time in some form or other; and although it is at 
present beyond the limits of our knowledge to explain how 
they arose, yet it is not right to conclude that the ctenophoral 
plates of Ctenoplana are degenerate or reduced structures 
merely because they are smaller than the ctenophoral rows of 
the Ctenophora. 
It is a groundless assumption to say that Cceloplana and 
Ctenoplana are modified creeping Ctenophores. Ctenoplana 
is an expert crawler, it is expert at hanging on to the surface 
film of water, and it is indeed an expert swimmer. Everything 
it attempts it does well in the old primeval fashion, and there 
is nothing degenerate about it. 
If Celoplana and Ctenoplana are neither Ctenophores nor 
Planarians, what are they? I think it is necessary to create 
a new order of Plathelminthes for their reception; and I 
propose to call the new order the Archiplanoidea, and to 
regard it as equivalent to the orders Turbellaria, Trematoda, 
Cestoda, and Nemertina. 
Furthermore, I should look to the Archiplanoidea for the 
ancestors of all the Plathelminthes (including the Nemer- 
tines) on the one hand, and of the Ctenophora on the 
other. 
The resemblance in form and shape, however superficial, 
between Ctenoplana and the Pilidium larva of Nemertines 
should not pass unnoticed; and it is a remarkable fact that 
the main axis of Pilidium passes through mouth and apical 
sense-organ as in Ctenoplana. 
In the Archiplanoidea, therefore, we have organisms pre- 
senting a transition from radial to bilateral symmetry. 
In the Cerianthide, as we know especially from the works 
