Note on the Pectoral Fin of the Sole, Achirus eapensis. 107 
locomotion at all similar to the pentadactyle limb. Attempts to trace any 
structural resemblance between a paired limb of any fish and that of a 
terrestrial animal cannot be said to have been much more successful. 
The theory of the origin of paired limbs from a lateral fold of the body 
has at least this in its favour — that its suggested function was essentially in 
the direction of an adaptation for an active pelagic life. The various 
arguments for and against this theory need not be repeated, and only some 
points noted which seem to indicate the origin of paired fins from vertical 
folds rather than lateral folds. 
(1) Vertical opercular folds of the epidermis exist in fishes at the 
present day, and there can be no doubt but that they existed in the primitive 
vertebrates, in which an expansible pharynx and its associated valvular 
opercula first appeared, not unlikely before balancing paired limbs suitable 
for a more pelagic life appeared, whereas, not only does no such longitudinal 
fold as the hypothesis requires occur in any living fish, but there is no 
satisfactory embryological or palaeontological evidence that it ever existed. 
(2) The function of the vertical folds is an obvious one at the present 
day, and even a necessity in the first primitive vertebrates, in which the 
respiratory current of water was drawn in by the mouth by means of an 
expansible pharynx provided with a framework of jointed branchial arches, 
whereas the suggested function of the lateral fold, a keel for steadying the 
body or a parachute-like expansion, are at the best conjectures, the first 
perhaps not mechanically justifiable. 
(3) That such opercular folds may attain greater dimensions is shown in 
some primitive sharks, and more especially in higher fish, in which they may 
have assumed the form of large organs taking on other functions, such as 
assisting in inspiration, acting as defensive and offensive organs or organs 
of locomotion. 
(4) It accounts for the absence of paired limbs in the Cephalochordata 
and the Cyclostomata in which no opercular folds are developed, and their 
presence, except when secondarily absent, in all vertebrates in which such 
folds appear. Grill septa provided with skeletal elements are present in the 
Cyclostomata as in other fishes, but there are no opercula and therefore (?) 
no limbs. 
A great many objections have been raised to G-egenbaur's theory as to 
the origin of paired limbs from branchial septa from two points of view, 
which may be distinguished from each other : first the suggested origin of 
girdles and radials from the skeletal parts of a gill septum, and second, the 
implied origin of the distal part of the limb from the gill flap or 
operculum. 
The objections to the first, which appear to be weighty, need not be gone 
into, but the objections to the second apply equally to the opercular origin 
and may be considered. 
