VI. A NEW CLASSIFICATION OF THE LYMNAEAS. 
GENERAL DISCUSSION. | 
The history of previous classifications of the Lymnzas shows that 
they 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- 
nea 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* 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 :* 
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). 
1Guide to Helices, p. XXVI. 
-) He of >, @. 4A il E 
