GENERAL CONSIDERATIONS ON THE MOLLUSC A. 331 



chambered shell of NavAilun, the highest development of the Mollus 

 can shell, was attained later. 



The head, in the Cephalopoda as in the Gastropoda, is well de- 

 veloped. The arms, which surround the mouth, at first sight appear 

 to helong to it, and yet, according to the results of more recent re- 

 search, another significance must be ascribed to them, i.e., they must 

 be regarded as parts of the foot. A fact which causes surprise and, 

 at first sight, is unfavourable to this view, is that some of the arms 

 shift to the dorsal side of the head, being here found behind the 

 mouth. This makes it difficult to understand how the foot could 

 have become transformed in this way, but when we sec how extremely 

 plastic it is, judging from different transformations which it has 

 undergone in the Lamellibranchia, the Prosobranchia, the Hetero- 

 poda and the Pteropoda, further modification is not so inconceivable, 

 especially when we find that the lateral parts of the foot in certain 

 Prosobranchia (Diotocardia) even develop tentacular structures. 



Another part of the foot of the Cephalopoda has at any rate be- 

 come changed into the funnel, which at first is paired. The pedal 

 character of this structure has never been doubtful, its origin being 

 at once betrayed by its position between the mouth and the anus. 

 The change uudergone by the foot in yielding the funnel is also very 

 great. We cannot here enter into the cpaestion as to whether, as has 

 been assumed, we have in this case to do with epipodia. The com- 

 parison of the different parts of the foot known as the propodium, 

 mesopodiiun, metapodium, parapodium and epipodium becomes very 

 difficult in modified forms, the inter-relationships of which alone pre- 

 sent great difficulties. The conditions of transformation and adapta- 

 tion may, in the various forms, have developed and modified very 

 different parts of the foot. 



In the Cephalopoda, the organisation of the Mollusca has attained 

 its highest development. In the structure of the adult the most far- 

 reaching differentiation has taken place and thus also in its ontogeny 

 we have the greatest complications found in the Molluscan stock, the 

 greatest departure being made from those features (total cleavage 

 of the egg, larval forms, etc.) which we have learnt to regard as 

 primitive in the lower as well as the higher representatives of the 

 Mollusca. 



