ANIMAL KINGDOM. 241 



exist between these animals, so different in every other re- 

 spect. 



The principal objection however which I have at pre- 

 sent to the above arrangement of the Mollusca, in which 

 I have almost entirely followed the learned Cuvier, is 

 that the genus Aplysia and his other Tectibranches are 

 more widely separated from the Nudibranches than their 

 general anatomy appears to allow. If they be united, the 

 Gasteropoda of Cuvier will evidently form a circular group, 

 as Dr. Leach has had the goodness to point out to me. But 

 however this may be, I have no doubt of there being 

 some great error yet undetected in the principles upon 

 which we are accustomed to arrange the Mollusca, and 

 that we shall never arrive at the truth either by looking, 

 like M. de Blainville, solely to the position and structure of 

 the organs of respiration, or, like M. Cuvier, to the method 

 of reproduction, as when he unites the Cyclobranches to 

 the Acephala. 



It may now seem invidious in me to indicate a proba- 

 ble remedy without showing, or at least trying its effi- 

 cacy ; but my inability at present to do either must be 

 an apology for stating my firm belief to be that we have? 

 in these curious animals, studied too little the nervous 

 system; and that the best way to group the Mollusca 

 naturally would be to follow up with more attention this 

 most important of the various parts of animal structure. 

 At all events, it is not to those collectors who are solely in- 

 tent on the external form of a shell or the streaks of co- 

 lour which ornament it, without the least idea of the 

 ftirm or structure of the animal by which it is inhabited, 

 that the hope of discovering the true arrangement of the 

 Mollusda can be held forth. The study of shells appears 



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