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 



