The stomach and bowels of the Chelonia, owing to the flattened form of the animals, are nearly as much laterally displaced as in the human subject. In the other reptiles they are not much out of symmetry, yet in none are they exactly symmetrical. The car diac end of the stomach tends, though often but a little, towards the left ; the pyloric is free, and can be brought without violence to the middle line, but yet it is always found 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 /cfrside of the latter. The single arterial trunk, with its branchial arches and the coalescing aortm, however, arc ,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 Pleuroneetidce, 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 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 nectidm 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.