ON CTENOPLANA. 339 
of Carl Vogt! and Ed. van Beneden,? we have also forms 
belonging to the category of radial animals, which are un- 
doubtedly physiologically radial, and nevertheless present a 
pronounced bilateral symmetry. 
Van Beneden’s views are clearly set forth in the following 
quotation from his memoir above quoted:—‘‘ Je partage 
entiérement l’opinion de Sedgwick et de Caldwell d’aprés 
laquelle le disque qui porte la bouche et les tentacles, chez 
les Actinozoaires, répond morphologiquement a la face neu- 
rale des Annéles, des Arthropodes, et des Chordés. Je pense, 
comme ces auteurs, que la bouche des Cnidaires est homo- 
logue 4 la fente blastoporique des Arthrozoaires. Les diver- 
ticules coelomiques qui sont, ontogéniquement parlant, la 
cause de la segmentation, répondent aux loges mésenteriques 
des Anthozoaires et les cloisons intersegmentaires sont ana- 
tomiquement équivalentes aux sarcoseptes.” 
If we accept these conclusions side by side with those 
derived from the study of Ctenoplana, we are compelled to 
frame the hypothesis, which I believe to be highly probable, 
of the diphyletic origin of Bilateralia. 
The following scheme will make this view clear, and will 
save a long discussion : 
TC tenophora. 
_A rchiplanoidea————__ 
ian __——Plathelminthes. 
Sheet Sn ee OO 
Cer Se ks 
ee —— CL lenin 
I believe this view will be found to be a natural one in every 
respect ; and if it be regarded by morphologists as substantiated, 
it will certainly relieve the science of morphology of several 
burdens. For instance, Hubrecht’s original speculations as to 
a relationship between the Nemertines and the Chordates, as 
well as Bateson’s comparison of the Nemertines with Balano- 
1 Carl Vogt, “ Des Genres Arachnactis et Cerianthus, f. Arch. de.Bioli 
t. ve 1888. 
* Hd. van Beneden, ‘“‘ Recherches sur le développement des Arachnactis,” 
ibid., t. xi, 1891. 
