204 J. GRAHAM KERR. 



It is because, at the present time, after many years of con- 

 troversy, the contrary view, which for shortness may be referred 

 to as the 'pedal' view, has gained the ascendency and has come 

 to be the one enunciated by the most authoritative text-books' 

 that the present discussion seems necessary. 



When Lankester published his Encyclopcedia Britannica 

 article on MoUusca, he pointed out that the view taught by 

 Leuckart, Huxley, and himself, that the Cephalopod arms are 

 pedal in their nature, was based upon three different sets of 

 evidence — to wit, those derived from 



(1) Their ontogenetic development ; 



(2) Their innervation ; 



(3) Their homology with the sucker-bearing processes of 

 the larval Pneumoderma. 



Of these (3) derived its force from the supposed pedal nature 

 of the sucker- bearing appendages. However, it has now been 

 satisfactorily shown' that they are purely cephalic, and there- 

 fore this argument, if it be argument at all, tells precisely in 

 the opposite direction. At present, therefore, the view that the 

 Cephalopod arms are parts of the foot rests upon (1) and (2). 

 In regard to (1), however, although it must be admitted that 

 the facts of embryology do tend to bear up the view that the 

 crown of arms is formed by an upgrowth from each side of the 

 foot, it must be borne in mind how extremely unreliable any 

 evidence, as to topographical relations, must be which is 

 based on the phenomena exhibited in the development of 

 enormously yolk-laden eggs. Therefore it appears that the 

 only one of the three classes of evidence adduced above which 

 can be considered of real weight, is that resting upon the 

 innervation of the parts under consideration, and that this 

 opinion is shared by other workers, is shown by its tendency in 

 more recent writings to supplant the evidence derived from 

 embryology. It appears, therefore, not inadvisable to submit 

 this portion of the evidence to a short critical examination, to 



' Lang's Lekrhuch, pp. 587, and Korschelt and Heider, p. 1176. 

 ' ' Challenger ' Reports : Pteropoda, Anatomy, p. 39. 



