Note on the Pectoral Fin of the Sole, Achirus capensis. 
103 
Whatever the explanation of the process may be, however, there is little 
doubt but that we have here the reduction of the pectoral fin to a simple 
fold of the epidermis of the body, lying parallel with and resembling the 
membranous epidermal border of the operculum. So striking is the resem- 
blance that the idea of their possible homology is suggested, though such 
resemblance is no proof of homology. The reduction of a pectoral fin to a 
lateral fold of the body also recalls one of the oldest theories of the origin 
of paired limbs, and is an inducement to inquire a little further into the 
matter. 
The simplest mechanism of respiration by gill slits would appear to be 
the establishment of a current of water by ciliated epithelium, as in some of 
the lowest vertebrates (Balanoglossus, Ascidians, Amphioxus). In the higher 
forms, the Cyclostomes, there are gill pouches connected by tubular passages 
to the exterior, where they open by small rounded apertures, guarded by 
more or less circular rings of cartilage. In the Elasmobranchs, which 
are provided with a more expansible pharyngeal cavity for inspiration, 
there is a simple mechanism consisting of a series of vertical folds which 
overlap the slit-like external openings of the gills, and act as effective 
valves, being mechanically closed in inspiration and opened in expiration. 
These are very well developed in some Elasmobranchs (Chalamydo- 
selachidae). In others (Chiniaeridae) there is a great development of the 
anterior of these flaps, which grows backwards over the gill openings, and 
acts as a valve for the whole of the branchial apparatus forming the 
operculum proper of the fishes. 
In bony fishes there is a further elaboration : this enlarged operculum 
strengthened by dermal bones takes over the functions of inspiration, while 
the membranous posterior border retains the original valvular function. 
Further, the bony operculum may come to subserve other functions, chiefly 
defensive, and, in exceptional cases (Anabas scandens), locomotory. We 
have therefore an illustration of how the simple epidermal flap which covers 
the external opening of a gill slit may be transformed into a somewhat 
elaborate organ, modified to perform other than its original functions. 
There is some reason to suppose that in the primitive vertebrates the 
number of gill slits were more numerous, and extended further along the 
body than in later forms. The reduction in the gill slits began probably in 
the more posterior ; they became closed up, and their gill arches and opercula 
disappeared, but it is quite possible that some of these opercula may, like the 
anterior operculum, have become developed into more elaborate organs 
taking on some other function, and should the evolution of the vertebrate 
body have been in the direction of a decrease in length and an increase in 
depth, the function now performed by the paired fins is a likely enough one. 
How the simple opercular flap could have tided over the critical period when 
the gill opening which it covered became closed up may be indicated by the 
