158 CONVERGENCE IN MINUTE STRUCTURE 



we find that the increase of brain - power is 

 merely an incident in evolution. 



It will be appropriate at this point to consider 

 another example of what I hold to be histo- 

 genetic convergence as between the lateral sense- 

 organs of the Polychaete family of the Capitellidse, 

 the abdominal sense-organs of Lamellibranchiate 

 Molluscs, and the lateral line sense-organs of 

 Vertebrates, thus involving three distinct phyla. 

 The lateral organs of Capitellidae are absolutely 

 comparable to the lateral line organs of Verte- 

 brata, but only by way of convergence, not by 

 way of homology. 



Let us first look at the abdominal sense-organs 

 of the bivalve molluscs, about which I am able to 

 speak with some authority inasmuch as, in one 

 instance, namely, that of the Windowpane Oyster, 

 Placuna placenta, I have found the true unpaired 

 abdominal sense - organ which had been over- 

 looked by previous investigators. The story of 

 our knowledge of these sense-organs is interest- 

 ing. The year 1881 saw a considerable advance 

 in the morphology of the Mollusca in consequence 

 of the publication of Spengel's paper on their so- 

 called olfactory organs, subsequently called the 

 osphradia by Lankester, and on the nervous 

 system. In dealing with the Lamellibranchiata 

 he was at first at a loss where to look for them, 

 but, thanks to a lucky chance (" einem gliicklichen 

 Zufalle "), on opening an Area noa he saw at once, 



