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NOTE ON THE PECTORAL FIN OF THE SOLE, ACHIBUS 
CAPENSIS: ITS ORIGIN AND SIGNIFICANCE . 
By J. D. F. Gilchrist. 
(With Plate IV.) 
The changes which take place in the body of flat fish in correlation with 
alteration of the position of the body are of particular interest from an 
evolutionary point of view. These changes are not so far back in the 
genealogy of the animal that we cannot hope for some light on the question 
of their origin, and the more recent such changes are the more instructive 
they will be. The following case may therefore be of special interest in this 
respect. It is also suggestive of some wider problems, such as the origin 
of paired limbs, on which some remarks are made. 
The pectoral fins of the soles have lost their original functions of balancing 
the body, and we find that their subsequent fate has been very diverse. In 
some species they become very much enlarged ; in others they disappear 
altogether. This is well illustrated in the South African soles, of which 
there is a great variety. Among these there is a small sole, Achirus capensis, 
which has been described as devoid of pectoral fins. It is common on the 
sandy shores of False Bay, and as it can readily be kept in confinement, it 
is a very suitable subject for the study of colour-adaptation to various 
surroundings. It was while some experiments of this kind were being- 
made that a peculiar action of the branchial aperture was observed, for, in 
inspiration, not only did the opercular membrane close down against the 
body, but a small flap of transparent skin was seen to rise from the body to 
meet it, thus forming a double valve for the closing of the aperture. Other 
living specimens were examined, and in all this little valvular membrane 
was found, apparently functioning as an accessory organ in respiration. 
On one occasion, however, a number of very small soles of this species was 
procured, and it was observed that the small flap, though present and well 
developed, lay applied to the body, and did not function in any way in the 
process of breathing. 
This structure may be best examined in an adult specimen. In one,. 
