ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA IQI 



the Lamellibranchiata.i jg modelled upon the archetype of the 

 Cephalous Mollusca. 



Such evidence as we possess with regard to the Brachiopoda, how- 

 ever, is purely anatomical, and (though I am aware that a great weight 

 of authority lies upon the other side) yet Mr. Hancock's opinion, that 

 they are rather to be considered as allied to the Polyzoa than to the 

 Cephalous Mollusca, seems to be quite as plausible as the more general 

 notion. 



Should this highly ingenious suggestion be found by embryology 

 to be correct, the Brachiopoda will have the same relation to the 

 Polyzoa as the simple Ascidians to the compound Ascidians, and will 

 form a parallel group to the former in M. Milne-Edwards's section of 

 " Molluscoides." 



In conclusion, I would observe that the archetypal Cephalous 

 Mollusk (as thus defined) is, in all its modifications, sharply sepa- 

 rated from other archetypes, whatever apparent resemblances or 

 transitions may exist. In all cases these will, I believe, on close 

 examination, be found to be mere cases of analogy, not of affinity. 



As Cuvier long ago remarked of the Cephalopoda and Fishes, so 

 we may say of the Cephalous Mollusca in general and other types : — 

 " Whatever Bonnet and his followers may say, Nature here leaves a 

 manifest hiatus among her productions." For instance, great as are 

 the apparent resemblances between a Lamellibranch and an Ascidian, 

 they all vanish upon closer examination.^ Neither in its anatomical 

 nor in its embryological relations does the branchial sac of an Ascidian 

 correspond with the mantle-cavity of a Lamellibranch. 



The nervous system is totally different. The three pairs of ganglia, 

 which exist in all Lamellibranchiata (even the apodal genera), are 

 replaced by one in the Ascidians, which is not homologous (as is 

 commonly asserted) with the branchial ganglion, or intersiphonic 

 ganglion of Lamellibranchiata, but with their pedal ganglion. 



The organization of the circulatory system is wholly different. The 



' The Lamellibranchiata are as truly cephalous as many Pteropoda, and the possession of 

 a distinct head is so much a question of degree as to be a very unfit classificatory character. 



^ I beg that I may not be misunderstood here. While I consider that there is no transi- 

 tion between the Cephalous Mollusca as such, and the Ascidians or Polyzoa, I also fully 

 believe (and so far as the Ascidians are concerned I have endeavoured to demonstrate. 

 Report on the Structure of the Ascidians, already referred to) that the archetype of the 

 Cephalous Mollusca, that of the Ascidians and that of the Polyzoa, are all referable to a 

 common archetype, the archetype of the Mollusca generally. It is one thing to believe that 

 certain natural groups have one definite archetype or primitive form upon which they are all 

 modelled ; another, to imagine that there exist any transitional forms between them. 



Every one knows that Birds and Fishes are modifications of the one vertebrate archetype ; 

 no one believes that there are any transitional forms between Birds and Fishes. 



