798 . FLEXURE AND ORIENTATION. 



of primitive topographical relations of parts, and perhaps a similar interpretation, although 

 not so certain, may be put upon the recurrent tentacular arteries (PI. LXXXII. fig. 5, 

 and PI. LXXXIII.''fig. 28). 



With regard to the siphuncle, while it is certain that it has a physiological relation 

 to the air-chambers of the shell, its morphological nature is not so clear, but I think 

 it is a vestige, a venniform appendix of A^'autilus, the remnant of the primitive elongate 

 body which we must suppose the remote ancestor of Nautilus possessed. The gut and 

 the coelom have vanished from the siphuncle, and it is now reduced to the condition 

 of a vascular appendage, of vital importance to JS'autilus in a physiological sense, but 

 a mere vestige morphologically. 



16. Flexure and Orientatiox. 



When squids are watched darting through the water with their pointed hinder 

 extremities turned in the direction of locomotion, the possibility of there being any 

 question concerning the orientation of the animal, i.e. concerning the true bearings of 

 the antero-posterior axis, never occurs to the observer. It is left to the refinements 

 of morphology to raise such a question, the discussion of which is only too apt to be 

 profitless and uninteresting. It is however impossible to neglect it in a work like the 

 present. 



Considered as Mollusca, the problem of the orientation of the body of Cephalopoda 

 depends upon and is created by two main features of organisation: — (1) the absence 

 of a plantar foot or platj^odium ; (2) the ventral position of the forwardly directed anal 

 aperture, which is associated \\*ith what Huxley (1853) called a neural flexure of the 

 intestine. 



The current notions regarding the orientation of a Cephalopod follow the lines laid 

 do^vn b}- Huxley, according to whom the visceral sac represents a dorsal hump or 

 abdomen while the arms represent a ventral foot, so that the morphological position of 

 the animal is taken to be that which is sometimes assumed for example by Octopus 

 (cf also my PL LXXVII. fig. 3) in which the arms are directed downwards, i.e. towards 

 the substratum and the visceral sac upwards. This is also the position which is usually 

 postulated for Nautilus on the tacit assumption that Nautilus in spite of its possession 

 of many primitive anatomical characters, affords no fresh clue in this regard. 



Another view with which I find mj-self, under certain reservations, in substantial 

 agreement, has been started in recent years by Dr B. Haller. I may quote the following 

 paragi'aph fi-om this author's work on Nautilus (op. cit. 1895, p. 191): " Bezliglich der 

 Orientirung mochte ich vorausschicken, dass ich den Nautilus, vde uberhaupt die Cephalo- 



poden (wie ich dies ausflihrlicher schon mitgetheilt habe' ) aus einem chitonartigen 



Mollusken so hervorgegangen mir vorstelle, dass letzterer mit seinem hinteren Korper- 



abschnitt sich von hinten nach vom und somit nach ventral warts bog, wodurch 



der After nach vom zu gelangte Vom ist der Kopf, hinten der Sipho." 



' Haller, B., " Studien iiber docoglosse und rhipidoglosse Prosobranchier, etc." Leipzig, 189-t, p. 149. The 

 "Sipho" means of course the siphuucle. 



