Bilateral Symmetry of the Ctenophora. 47%. 
point.” If this remarkable form of the Cestwn Veneris might: 
furnish the inducement to a new investigation of its right to 
stand as a radiate animal, it cannot, however, be made available 
as evidence to the contrary, any more than the globular form of 
a rolled-up Spheroma can exclude that animal from the bilateral 
series. If the Ctenophora be regarded as biradiate animals, this 
ribbon-like form, moreover, loses all its remarkableness ; Cestum 
then places itself in the neighbourhood of the Cydippe with a 
circular transverse section, in exactly the same way as the long- 
rayed Asteriade and Ophiure take their place in the neighbour- 
hood of the globular Echinus. 
A second reason for the assumption of a “bilateral symmetry ” 
appears to have been furnished by the duality of. various parts, 
such as the orifices of the funnel, the oral lobes, the gastric ves- 
sels, tentacular filaments, &c. ‘Even in the otherwise radiately 
constructed Beroés”’ Gegenbaur finds “the bilateral symmetry 
indicated” in the two orifices of the funnel*, and supposes the 
two tentacular filaments of Cydippe to be arranged “in accord- 
ance with bilateral symmetry}.” It is true that most of the 
arts of bilateral animals are present in duality; but the distri- 
ation of these duplicate parts, in the Ctenophora, upon two 
planes perpendicular to each other, far from being a proof of 
bilateral symmetry, is rather something perfectly irreconcileable 
therewith, and, combined with the quadruplication of all parts 
exterior to these planes, is a certain characteristic of biradiate 
arrangement. However, quite independent of the characters of 
radiate and bilateral animals stated above, it is a matter of 
wonder that the contradiction which lies in regarding the orifices 
of the funnel and the tentacular filaments as both bilaterally: 
symmetrical has escaped notice. If it be the orifices of the 
funnel, then, in Mnemia, for example, the narrow sides and oral 
lobes lie right and left, the broad sides with tentacular filaments ¢ 
and gastric vessels above and below. If it be the tentacular 
filaments, the broad sides and gastric vessels are right and left, 
the narrow sides, oral lobes, and orifices of the funnel above 
and below. One supposition reduces the other ad absurdum. 
In both suppositions, moreover, in contradiction to the most 
essential characteristic of bilateral structure, there is no distinc- 
tion of dorsal and ventral surfaces. 
A further remark of Gegenbaur’s has always been unintelli- 
gible to me. In the Ctenophora the radiate type of the Coelen- 
terata is said to pass over into the bilaterally-symmetrical type 
“by a preponderating development of the individual parts taking 
* Wiegimann’s Archiv, xxii. p. 170. t+ Loe. cit. p. 176. 
} These are indeed very minute, but not wanting, in Mnemia Schweig- 
geri, Eschsch, 
