1899] CONSIDERATIONS CONCERNING SYMMETRY 105 
significance of some of these ganglia has been satisfactorily learned. 
The sense organs bear out apparently the statement that vertebrates are, 
speaking generally, bilateral animals, The pineal eye, and the arrange- 
ment of the sense organs in some invertebrate types, may be cited as 
being favourable to other views of symmetry. It will be remembered 
that C.S. Minot thinks that the cerebral ganglia of a worm may fairly 
be regarded as the optic central organs, and that some of the sub- 
oesophageal would do for cerebral ganglia if the mouth were further 
back. 
Asymmetry. 
The five fingers and the five nerves that form the brachial 
plexus have been associated by some anatomists (Paterson), but 
Bardeleben has given reasons for regarding the primitive hand as 
having a much larger number than five digits. The Gasteropods show 
rare examples of asymmetry. The left respiratory organ and the left 
kidney in part lose their character, and the right organs do the work 
of the pair. Mechanical causes seem to be the main agents in bring- 
ing about the absorption of the absent organs. A superficial bilateral 
symmetry appears in some, but not only is there want of dorso-lateral 
symmetry, but the dorsal growth of the animal has been so consider- 
able, and the form has become so altered dorsally and ventrally, that 
with the exception of a portion of the body in front, it is impossible 
to see an approach to quadrilateral symmetry. There are, however, 
the four ganglia or six, which may be looked upon as part of a radial 
quadrilateral or hexagonal symmetry. The renal organs of the lancelet 
are sometimes asymmetric. The newly-hatched sole is symmetric ; the 
size is 3°55-3°75 mm. long. This creature swims with its yolk sac 
up because the latter is light (Cunningham). The eyes come to lie 
on the upper surface (the right). Remembering that if a fish is to 
forage and rest on the floor of a bay, it must be spread out laterally or 
have some supporting apparatus in connection with its fins, it seems 
natural that the sole or plaice, not being able to make suitable pro- 
vision in either of these ways, should simply lie on its side and turn 
its second eye up. The result is advantageous in this way, that a 
surface of one to two square feet is presented to the view of a voracious 
dog-fish, skate, or shark, so that the apparent size may save the sole or 
plaice. The asymmetry is, therefore, susceptible of a triple explanation. 
The diminution of one lung in snakes is due to the elongation of the 
body; with the elongated lung a certain amount of dislocation of the 
viscera is associated. The single lung is, under the circumstances, better 
suited for respiration. The single ovary in birds is most convenient 
in consequence of the large eggs, and the large ovary is connected with 
the persistence of the abdominal rather than the chylopoietic aorta. 
The latter is, evidently, the best for mammals. Asymmetry in the 
dolphin tribe is marked in the skull. The large left upper canine 
