FLEXURE AND ORIENTATION. 799 



In the course of the following pages I shall give my own reasons for regarding 

 the cephalopodium of Nautilus as being anterior in position and (far from being ventral) 

 having a marked dorsal inclination by which, as mentioned above, its main axis as 

 determined by the direction of the sheaths of the digital tentacles has become tilted 

 at an angle with the skeletal axis (PL LXXXI. figg. 1 and 2) 1 . 



With regard to the flexure or coiling of the external, multilocular, involuted shell 

 of Nautilus, it was noted by Huxley (1853, p. 49) that the direction in which it is 

 wound is the same as that in which the intestine is bent, so that if the coils of the 

 shell be imagined free as they are in Spirula, the convexity is dorsal and the concavity 

 ventral. This method of coiling has been called exogastric by Owen 2 , while the shell 

 of Spirula which is coiled in the reverse direction is endogastric. Spirula is a coiled 

 Belemnite (Valenciennes), Nautilus a coiled Orthoceras (Owen), and Ammonites a coiled 

 Baculites (Owen). 



The shell of Nautilus is external and protective, it is an involute cone of great 

 length, and its length increases pari passu with the growth of the animal until the 

 latter reaches- its specific cubic capacity or limit of growth. The deposition of septa 

 and secretion of gas convert the shell into a hydrostatic apparatus. 



Owen regarded the straight chambered shells of fossil Cephalopoda as having been 

 produced by the progressive unwinding of a coiled shell and the evidence seems to point 

 in this direction. 



If this be so, then the origin of the shell is lost in the remotest antiquity, and 

 it is useless to discuss the question whether the arthrocochlid (von Jhering) shell-system 

 of the Polyplacophora (Chiton) has any possible relation to the septate shell of the 

 primitive Cephalopoda. 



The actual facies of Nautilus is determined by its visceral flexure, and by its 

 pedal and epipodial topography, and this is therefore the place to refer to a question 

 which has been raised by Dr Plate 3 , namely, as to whether the siphonopodium (Schwimm- 

 fuss) of Cephalopoda is derived from a platypodium or vice versa. It seems to me that 

 the direct answer to this question depends entirely upon one's individual proclivities. 

 The cleft funnel of Nautilus conveys the impression of a platypodium, with the margins 

 folded over each other in a manner recalling that in which the large extensile foot 

 of certain Gastropods (Harpa, Oliva, etc.) is capable of wrapping itself round foreign 

 objects. 



If we imagine the body of Nautilus to be straightened out and the anus carried 

 back to its primitive terminal position, the dimensions of the primitive platypodium, 

 extending from end to end of the body, may appear appalling. It is however quite 

 conceivable that the enormous bulk of the Cephalopods, as a whole, was correlated with 

 the achievement of the peculiar relations of the animals to their shells, and therefore 

 came into being subsequently to the evolution of the plan of composition of the 

 Cephalopod organisation. 



1 I may also refer the reader to the two preceding chapters. 



2 Owen, E., " On the relative positions to their constructors of the chambered shells of Cephalopods." 

 P. Zool. Soc. London, 1878, pp. 955—975, PI. 40. 



s Plate, H. L., op. eit. 1901, p. 559. 



