SYMMETRY. 



849 



leaning to the right. This renders the spleen 

 more conspicuous on the left. The liver of* 

 reptiles extends from side to side, but the 

 right lobe is the largest.* 



In the tadpoles of toads and frogs, I have 

 observed that there is no gill opening on the 

 right side. In the Lepido-siren the anus is 

 situated on the right side of the mesial ridge 

 of the tail. 



Fishes. The heart of fishes is posited 

 symmetrically, but the relative position of the 

 auricle and ventricle is usually unsymmetrical, 

 the former being behind and to the /t/rside of 

 the latter. The single arterial trunk, with its 

 branchial arches and the coalescing aortas, 

 however, are .precisely alike on both sides. 

 The intestinal canal, being generally much 

 longer than the abdomen of the fish, is neces- 

 sitated to throw itself into unsymmetrical 

 loops and convolutions. 



There is a very remarkable departure from 

 symmetry in all the members of the group of 

 fishes called Pleuroncctidcc, or flat fishes, such 

 as the turbot, halibut, sole, and flounder. 

 These fishes lie at the bottom, and swim, on 

 one side ; and the side which they keep 

 uppermost is coloured dark like the back of 

 other fishes, whilst that which they keep 

 undermost is white like another fish's belly. 

 The dark-coloured side is also somewhat 

 convex, whilst the white side is nearly flat. 

 The dorsal azygos fin is continued on to the 

 head, beyond the eyes, almost to the muzzle, 

 and, what is most remarkable of all, both the 

 eyes appear on the dark side, and are actually 

 both situated on one side of the dorsal azygos 

 fin. With the exception of the different co- 

 lour of the skin, the different degrees of con- 

 vexity of the two sides, and a slight distortion 

 of the mouth, the whole of these fishes are 

 symmetrical in all their parts, besides the eyes 

 and the structures immediately surrounding 

 them. The bones immediately contiguous to 

 the eyes suffer the following remarkable dis- 

 tortion : The occipital bone is almost per- 

 fectly symmetrical, and its mesial crest is 

 continued far forward as a sagittal crest be- 

 tween the parietals in the real as well as the 

 apparent middle line, supporting the styliform 

 bones to which the rays of the dorsal azygos 

 fin are articulated* There is a slight dif- 

 ference in the size of the parietals, that of the 

 right or white side being the largest ; but it is 

 the frontal and pre-frontal bones that suffer the 

 greatest distortion. Arriving at the frontals, 

 the real or primordial middle line is suddenly 

 deflected towards the left or dark side, whilst 

 the sagittal crest, still supporting the azygos 

 fin, is continued straight on, on the right fron- 

 tal alone. The left frontal is less expanded, 

 but more clumsy than the right, and extends 

 forwards in a curved form between the eyes, 

 presenting a concavity towards the right. "To 

 this concave curve is sutured a falciform pro- 

 cess produced from the left anterior corner of 

 the right frontal, which is quadrate. This 

 suture of course indicates the real middle line. 

 The eyes are situated on each side of the sep- 

 * Vol. IV. fig. 220. p. 304. art. REPTILIA. 



VOL. IV. 



turn thus formed. The end of this compound 

 septum rests on the suture between the two 

 pre-frontals, and this suture, which again in- 

 dicates the real mesial line, presently regains 

 the apparent middle. The right pre-frontal is 

 much larger than the left, and comes again 

 into contact with the right frontal on the 

 outside of the right eye, which therefore oc- 

 cupies an orbit with a complete bony margin, 

 whilst, the same thing not occurring on the 

 left, the eye of that side has no orbit, but 

 seems to lie loose in the soft structures 

 of the cheek. The symmetry of the base 

 of the skull is disturbed but slightly, the 

 long sphenoid and vomer forming nearly a 

 straight line, and participating but slightly 

 in the abrupt deflection of the middle line 

 which takes place above. Not the least curi- 

 ous part of this history is the non-participa- 

 tion of the dorsal azygos fin in the deflection 

 of the mesial line : it furnishes an additional 

 proof that its rays are not a part of the endo- 

 skeleton. The eyes of the pleuronects are of 

 different sizes ; the furthest from the dorsal 

 fin (the left, the one that has no orbit) being 

 the smallest, and the optic nerve and optic 

 lobes of the brain, which belong to it, are 

 smaller than their fellows.* 



In using the terms right and left in the 

 above descriptions, I have constantly had in 

 view the turbot, which is coloured, and shows 

 its eyes on the left side ; but the sole, dab, 

 and flounder, are coloured on the right, and 

 therefore those terms must be reversed when 

 applied to them. It is extremely common to 

 meet with individual specimens of Pleuro- 

 nectida? coloured, so to speak, on the wrong 

 side, that is to say, not on that side which is 

 usual, and the rule for the species. Turbots 

 coloured, and having their eyes on their right 

 sides, are frequently met with, and the flound- 

 ers brought to the London market are almost 

 as frequently coloured, and show their eyes, 

 on the left as on the right side. The "un- 

 symmetrically posited intestines of these fishes 

 do not participate in this transposition, but 

 occupy, respectively, the same sides in the 

 monstrous as in the normal individuals. 

 The frequency of these monstrosities tempts 

 one to conjecture that external circumstances 

 may, perhaps, determine which side of the 

 pleuronect shall have the eyes and be coloured. 



Articulata. The animals composing this 

 sub-kingdom are bilateral and symmetrical. 

 The abstract pattern or type of an articulate 

 animal, like that of the Vertebrata, is a sym- 

 metrical figure. f From this primordial sym- 

 metry there are but few deviations, and those 

 exceedingly trifling in amount. 



Amongst the Entozoa, the English tape- 

 worm (TtEnia solium), if the flat sides be 

 taken as dorsal and ventral, presents an in- 

 stance of a-symmetry in the position of its 

 genital orifices, which are situated on the 

 edge of each joint, sometimes on one side, 

 sometimes on the other, indifferently. 



In the Natural History series of the Mu- 



* See fig. 409. Vol. III. 

 | Vol. I. Fig. 378. art. CRUSTACEA. 

 3 i 



