CHANGES OF FUNCTION, ORGANS AND TOPOGRAPHY. 797 



and substitution of organs are often found associated with more or less radical changes 

 of topography. 



One of the simplest examples of change of topography is to be met with in connection 

 with the position of the vent in Teleostean fishes, even in closely allied species. Instances 

 of this kind (as also in regard to the position of the pelvic fins of fishes) are exceedingly 

 numerous, but one of the most striking cases has been brought to my knowledge by 

 the courtesy of Mr G. A. Boulenger, F.R.S. In the Berycid genus Trachichthys the vent 

 is placed normally behind the series of abdominal scales, but in the closely allied genus 

 Paratrachichthys "as first observed by Giinther, the vent is far forward between the 

 ventral fins, in front of and not, as usual, behind the series of abdominal scales 1 ." 



I have selected this example of change of position of the vent, because the antero- 

 posterior axis of many animals corresponds more or less closely with what has been 

 called the oro-anal axis; it does so for example in the most primitive group of Mollusca, 

 the Amphineura (Chiton, Ckaetoderma, Neomenia). In consequence of this circumstance 

 the oro-anal axis has frequently been adopted as a fixed quantity, from which all bearings 

 must be taken. This may be true in special cases but it is clearly not true for the 

 Cephalopoda, at least this is my opinion of the matter, and I do not regard the antero- 

 posterior axis as synonymous with the oro-anal axis, although, as indicated above, I do 

 not forget that the two may coincide. 



In Nautilus and the Cephalopoda generally, in consequence of the flexure of the 

 intestine there is no longer any question of an oro-anal axis, but there is an antero- 

 posterior axis which it is our business to discover. 



The evidence of change of topography in Nautilus based upon anatomical conditions 

 is to be traced directly in the occurrence of conflicting axes, and in the existence of 

 recurrent nerves and arteries, and indirectly in the relations of the siphuncle. With 

 regard to the conflicting axes I am aware that these are visionary allies, since they 

 involve an imaginary interpretation of imaginary lines, but I have come to the conclusion 

 that what I have to say is not destitute of possible interest. 



The capito-pedal cartilage, the skeletal basis of the body of Nautilus, possesses 

 a distinct longitudinal axis of its own, and this axis does not coincide with the axis 

 of the cephalopodium, but describes an angle with it 2 (PI. LXXXI. figg. 1 and 2). 



It seems a possible view to take that the skeletal axis may represent the primitive 

 longitudinal axis of the uncoiled ancestor of Nautilus, that the tilting of the cephalopodial 

 axis is evidence of the concrescence and translocation of the primitive epipodium (the 

 tentacles of Nautilus being here regarded as epipodial tentacles), and finally that the 

 ventral pallial flexure of Nautilus has resulted in the formation of a visceral sac or 

 abdomen, which lies chiefly below the level of the primitive antero-posterior axis 

 (PL LXXXI. fig. 2). The lateral portions or cornua of the cartilage which determine 

 the direction of its main axis, occur along the angle of junction of the epipodium 

 (cephalopodium) and protopodium (funnel). 



The recurrent nerves of the posterior ophthalmic tentacles obviously indicate change 



1 Boulenger, G. A., "Notes on the classification of Teleostean Fishes. II. On the Berycidae." Ann. Nat. 

 Hist. (7) ix. 1902, p. 203. 



2 These relations are best seen in dissections of relaxed specimens. 



104—2 



