ON CTENOPLANA. aol 
about eighteen. But I should hesitate to insist upon this as a 
specific difference. 
9. General Conclusions.—With regard to the systematic 
position of Ctenoplana, which we now know to be an adult 
animal, I am strongly of opinion that it is an ancestral form, 
and not, as some zoologists seem to suppose, a highly modified 
ereeping Ctenophore. By ancestral form I simply mean a 
primitive archaic form belonging to an ancestral type, and of 
course I do not imply that it is the actual ancestor of any- 
thing in the world. That the Planarians and Polyclades in 
particular have close affinities with the Ctenophora there can 
be no doubt, but it is very much open to question whether 
the former are derived from the latter. The view that the 
Polyclades are so derived seems a reversal of the natural order 
of events, which point to the littoral fauna as the origin both 
of the pelagic and of the abyssal fauna. 
Are we to regard the immediate ancestors of the Turbellaria 
as amorphous forms, like Trichoplax, or forms without any 
kind of symmetry, like Planule or the Mesozoa? Or, on the 
contrary, are we not rather to regard their immediate ancestors 
as forms with some kind of radial symmetry ? 
Having regard to the complete bilateral symmetry of the 
flat-worms, and more particularly their well-developed nervous 
system, with cerebral ganglion in even the lowest forms, I 
cannot imagine them to be derived directly from amorphous 
organisms, but rather from animals which possibly, like Cteno- 
plana, possessed a biradial symmetry. 
Ctenoplana approaches more nearly to a condition of bilateral 
symmetry than the Ctenophores do, in that it possesses very 
clearly differentiated dorsal and ventral surfaces. And this is 
exactly what we should expect to find in the littoral or sub- 
littoral ancestor of such purely pelagic forms as the Cteno- 
phora, the pelagic habit, as is well known, often tending to 
produce a more or less radial symmetry. 
tentacle sheaths, and the “system of canals” are the sheaths themselves, 
which do in fact send off occasional diverticula, possibly due to the contraction 
of their walls during preservation (ef. figs, 5 and 10). 
