VI. A NEW CLASSIFICATION OF THE LYMN^EAS. 



GENERAL DISCUSSION. 



The history of previous classifications of the Lymnaeas shows that 

 thev have all been based on characteristics of the shell, the internal or- 

 gans have been almost totally neglected in providing data for this 

 purpose. The shell is more directly and easily modified by external 

 influences than are the internal organs and, hence, is a more variable 

 and less stable criterion for purposes of nomenclature. This is es- 

 pecially true of fresh-water mollusks, and is notably apparent in Lym- 

 ncra where a single species in a given locality may present variations 

 characteristic of the majority of the recognized groups which have 

 been founded upon shell characters. 



Classifications based on the modifications of a single organ, as 

 the shell, jaw or radula. are predestined to failure, because of the great 

 changes which have taken place in these organs in closely related 

 groups. It has been well stated by Dr. Pilsbry 1 that "a classification 

 which takes cognizance of several totally diverse, uncorrelated organs, 

 is more reliable than one based upon a single organ ; for the reason 

 that while some one organ or system of directly correlated organs, may 

 independently assume similar forms in members of different stocks or 

 phyla, when they are subjected to similar conditions of life, the prob- 

 abilities are remote that several organs not directly correlated will be 

 simultaneously so modified. Again, the ancestral form of a certain 

 organ may be retained in several groups widely diverse in other re- 

 spects ; and moreover, the taxonomic value of a given structure varies 

 widely in different families and genera." This statement, it would 

 seem, applies quite as well to the fresh-water pulmonates as to the land 

 mollusks. 



Pilsbry has also stated that a natural classification of the pul- 

 monates should be based on the following organs : 2 



Organs of protection (shell, mouth, integument of body). 



Organs of locomotion (foot with pedal-grooves, tail gland, etc.). 



Organs of reproduction (genitalia, comparative size of eggs, etc.). 



Organs of nutrition (jaw and teeth, intestinal tract, kidney). 



^uide to Helices, p. XXVI. 

 *1. c. p. XXVII. 



