ROBSON : STYLE-SAC AND INTESTINE IN MOLLUSCA. 45 



misleading. Thus, from Matthias' account ^^ we find that certain 

 Eulamellibranchs agree with the Protobranchia in having the style- 

 sac in open communication with the intestine, while a Filibranch like 

 Phaseolicama has a separate sac ; and the Septibranchia apparently 

 are in the same condition with regard to this character as the Proto- 

 branchia. This, of course, leaves us with two alternative conclusions, 

 either that the taxonomy of the Lamellibranchia is as far off as ever 

 from a rational order, or that, as several authors have suspected, 

 there has been independent evolution within the various groups. 

 I hope to discuss these alternatives in another place ; but in the 

 meantime, while we are confronted with such a dilemma, we can only 

 content ourselves by pointing oiit that the Protobranchia which 

 are clearly the most primitive do not have a differentiated style-sac, 

 and that they agree therein with the more primitive Gastropoda. 

 That a good deal of independent evolution takes place in the 

 smaller groups is evident from a comparison of Paludestrina 

 and Bythinella among the Prosobranchs. But, having regard 

 to the issue raised by Ghosh's suggestion, the most important 

 point in the morphological series is the one in the Lamelli- 

 branchia and Prosobranchs, where we find the style-sac un- 

 differentiated. Whichever course was followed by this structure 

 in its evolution, we may safely assert in conclusion that 

 there has been a remarkable and close parallelism between the 

 Gastropoda and Lamellibranchia. We may, at this point, recall 

 that a pyloric coecum is present in the Scaphopoda, though whether a 

 style is secreted in it is very doubtful. Even if we may not bring 

 the Scaphopoda into the argument, the remarkable similarity 

 between Gastropoda and Lamellibranchia with regard to the 

 evolution of the style-sac (a similarity called homoplasy by 

 Lankester) is another instance of the fundamental unity that 

 characterizes the Mollugca. 



Many authors have constructed genealogical trees illustrating the 

 relationships of the classes of Mollusca. I do not wish to add yet 

 another sapling to that adventurous plantation. But I think 

 we may allow ourselves the following conclusions from these 

 observations : — 



(1) The Gastropoda and Lamellibranchia which otherwise 

 suggest by their structure a very remote ancestral point of separation 

 have in respect of their digestive system retained in common (a) 

 a singularly characteristic .structure and (6) equal developmental 

 potentiality with regard to it ; and 



(2) That with regard to the Lamellibranchia the general 

 morphological importance of the style-sac as set forth above renders 

 that structure an important factor in the classification of that group 

 as Matthias has suggested. It has yet to be seen whether our 

 taxonomy is wrong and the class requires regrouping, or whether 

 there has been independent evolution on a large scale leading 



