796 CHANGES OF FUNCTION, ORGANS AND TOPOGRAPHY. 



pleural folds bearing tentacles on their margins. Anteriorly the two borders of each 

 epipodium merge together, and the single tentaculiferous lobe encroaches in a very 

 singular manner over the dorsal side of the mouth between the latter and the cephalic 

 tentacles. There is also a nuchal membrane which arches over the head in front 

 (PI. LXXVI. figg. 4 and 5). 



When the protopodium presents a Hat or plantar surface of reptation as in Haliotis 

 and most Gastropoda I shall speak of it as a platypodium. The platypodium is there- 

 fore to be defined as a plantar protojaodium. 



The Cephalopoda and the prosobranchiate Gastropoda agree in so many features of 

 organisation that a definite relationship between them seems to be highly probable. 

 Perhaps the most fundamental difference lies in the absence of a plantar foot in the 

 Cephalopoda. 



In a subsequent chapter I shall endeavour to show that the cephalopodium oi 

 Nautilus is of epipodial origin, while the siphonopodium represents the protopodium alone. 



It is true that there is a discussion among malacologists concerning the true nature 

 of the epipodium, some authorities, foremost amongst whom is Dr Pelseneer, maintaining 

 the view of its pedal origin, whilst M. Boutan' is in favour of its pallial origin, and 

 calls it the " manteau inferieur." I would, however, derive the cephalopodium of Nautilus 

 from a primitive epipodium wholly irrespective of any morphological interpretation which 

 may be put upon the latter. 



I may nevertheless suggest or rather repeat a suggestion which is implied in a 

 paragraph of Huxley's work (1853, p. 50)^ that the epipodium is a pleural fold of the 

 body-wall, and perhaps should be discussed independently of the foot as well as of the 

 mantle. 



In the work of comparing Nautilus with other Mollusca, but especially with a Gas- 

 tropod like Haliotis, everj-thing depends upon the interpretation of pallial, pleural, and 

 pedal derivatives, or in other words, we have to ascertain the relations and modifications 

 of the typical molluscan mantle, epij)odium, and protopodium respectively. 



15. CHANGE.S OF FUNCTION, ORGANS AND TOPOGRAPHY. 



The doctrine of Change of Function (Functionswechsel) was, I think, first raised 

 to the rank of a moi-phological principle by Dr Anton Dohrn in 1875. The development 

 of organs by substitution or " Organwechsel " was first made the basis of a principle 

 by Kleinenberg in 1886. I am not aware that the phenomenon of translocation of organs 

 has been hitherto utilised for the purpose of establishing a principle of " Change of 

 Topography " which should have equal value with the other two in morphological inquiry. 

 The three principles may operate severally or in combination. Both change of function 



' Boutan, L., "Le syst^me nerveux du Parmnphorus (Scntus) danB ses rapports avec le manteau, la coUerette 

 (manteau inferieur) et le pied." Rev. biol. Nord France, ii. 1890, pp. 449 — 478, PI. vni. and ix. 



2 Huxley says the bilateral archetj-pe of the Mollusca corresponds with the vertebrate and articulate 

 archetypes. " The appendicular system of the Vertebrata and Articulata is represented by the epipodium in 

 the Cephalous Mollusca." 



