396 
F. M. BALFOUR. 
shows, if true, that the ventral and median position of the 
mouth in many Turbellaria is the primitive one. 
The above suggestion as to the mode of passage from the 
radiate into the bilateral form differs entirely from t hat 
usually held. Lankester,^ for instance, gives the following 
account of this passage : 
Pig. 15 . — Diagram of a Larva of the^ Dolyzoa. m. mouth ; an. anus 
st. stomach ; s. ciliated disc. 
It has been recognised by various writers, but notably 
by Gegenbaur and Haeckel, that a condition of radiate 
symmetry must have preceded the condition of bilateral 
symmetry in animal evolution. . The Diblastula may be 
conceived to have been at first absolutely spherical with 
spherical symmetry. The establishment of a mouth lead 
necessarily to the establishment of^a structural axis passing 
through the mouth, around, which axis the body was 
arranged with radial symmetry. This condition is more or 
less perfectly maintained , by many Coelenterates, and is 
reassumed by degradation of higher forms (Echinoderms, 
some Cirrhipedes, some Tunicates). The next step is the 
differentiation of an upper and a lower surface in relation to 
the horizontal position, with mouth placed anteriorly, 
assumed by the organism in locomotion. With the differ- 
entiation of superior and inferior surface, a right and a left 
side, complimentary one to the other, are necessarily also 
differentiated. Thus the ‘organism becomes bilaterally 
symmetrical. The Coelentera are not wanting in indications 
of this bilateral symmetry, but for all other higher groups 
of animals it is a fundamental character. Probably the 
development of a region in front of, and dorsal to the mouth, 
forming the Prostomium, was accomplished pari passu with 
the development of bilateral symmetry. In the radially 
a form described by Kowalevsky, ‘ Zoologischer Anzeiger,’ No. 52, p. 140, 
as being intermediate between the Ctenophora and the Turbellaria. There 
does not, however, appear to me to be sufficient evidence to prove that this 
form is not merely a creeping Ctenophor. 
^ ‘Quart. Journ. of Micr, Science,’ vol. xvii, pp. 422-3. 
