On the Supposed Bilateral Symmetry of the Ctenophora. 475 
Couch’s ‘Cornish Fauna’ but the Fredericella Sultana, which 
has been found near Penzance. There can be little doubt, how- 
ever, that many of the species might be discovered by careful 
examination, 
. EXPLANATION OF PLATE XVI. 
Fig. 1. Mimosella gracilis, Hincks, nat. size. ; 
Fig. 2. Arachnidia hippothooides, n.sp.,magnified : 2a, a single cell, more 
highly magnified. 
Fig. 3. Discoporella flosculus, n. sp., nat. size and magnified : 3a, side view 
_ of cells; 35, one of the tubular orifices in the centre of the disk. 
LII.—On the supposed Bilateral Symmetry of the Ctenophora. 
By Fritz MULiER*. 
In radiate animals we can distinguish only the front from the 
back, or the top from the bottom; in bilateral animals we can 
simultaneously distinguish the front from the back, and the top 
from the bottom. Radiate animals are divisible into symmetrical 
parts through as many planes as there are rays present ; bilateral 
ones only into symmetrical halves through a single plane: ra- 
diate animals have an axis at the intersection of the above planes; 
bilateral ones only the median plane, and no axis. In radiate 
animals only the parts situated in the axis can be present singly; 
all the parts in the middle and on the borders of the rays are 
repeated to the number of the rays, all the other parts to twice 
this number. In bilateral animals all parts situated in the me- 
dian plane may occur singly, and all parts out of this plane exist 
in pairs. 
the divisional planes of the rays be allowed to turn round 
the axis, retaining their relative position, the animal will con- 
stantly be cut into congruent parts; bilateral animals. are not 
divisible into congruent parts. Each individual ray of a radiate 
animal is bilaterally symmetrical ; bilateral animals are not divi- 
sible by planes parallel to their longitudinal direction into frag- 
ments which are again bilaterally symmetrical. 
When the rays are in pairs, therefore, in 2-, 4-, or 6-rayed 
animals, every plane passing through the axis cuts the body into 
congruent halves, and each of these sections is again cut through 
the axis into congruent halves. Bilateral animals (as also Ra- 
diata with an uneven number of rays) are not divisible into 
congruent halves; a right half cannot be replaced by a left one, 
nor ean an entire animal be made out of two right halves of 
congruent animals. If, on the other hand, two even-rayed ani- 
mals were cut in the same way into congruent halves, any two 
* Translated by W.S. Dallas, F.L.S., from Wiegmann’s Archiv, 1861. 
