LYMNZIDA® OF NORTH AMERICA. ve 
prostate. Both ova and sperm are produced in the same follicle of the 
ovotestis, the sperm ripening first and being stored in the spermatheca 
to await the expulsion of the ova. 
There is considerable modification of the form of the sexual 
organs among the various species of Lymnzas. Curiously enough 
this modification is confined almost entirely to the male system, the 
female system remaining practically unchanged. The penis and 
penis-sac exhibit many modifications as does also the prostate. 
These modifications being constant and uniform in certain groups 
of species, have been found very useful in preparing a classification 
of the pond snails. The comparative length of penis and penis-sac, 
for example, varies from one-quarter in Lymnea stagnalis to one 
and one-quarter in Radix auricularia. (PI. IV, fig. 1.) Care must 
be used in making these comparisons to be certain that the penis is 
pulled out to its fullest extent. Frequently it is partly inverted in 
the penis-sac and presents an appearance as shown in figure G, 
plate IV. The retracter and protractor muscles exhibit great 
variation, even in individuals of the same species (see plates XIV, 
XV). 
If the opinion now held by leading malacologists is true that 
the hermaphrodite condition is secondary, then the Lymnzas must 
be considered of very ancient lineage and older in time than the 
Pulmonata Stylommatophora, in which the male organs have been 
more completely superimposed upon the female individual and in 
which there is a common genital orifice. A comparative study of 
the genitalia of all the Pulmonata Basommatophora would doubt- 
less reveal many modifications of the sexual organs bearing on this 
point. A more detailed discussion of the modifications of the geni- 
talia will be found in the chapter on classification. 
e. THE NERVOUS SYSTEM, Plates I and V. 
The nerve centers of Lymnza with their connections belong to 
the type of nervous system known as Euthyneurous, in which the vis- 
ceral nerve cords remain straight. The nerve centers form a ring or 
nervous collar surrounding the cesophagus, immediately posterior to 
the buccal sac (plate I, 47). In this nerve ring the two cerebral gan- 
glia are above the cesophagus and all of the others are below. There are 
nine main nerve centers, as follows: two CEREBRAL, two PLEURAL, two 
VISCERAL, tWO PEDAL and one ABDOMINAL. In addition there are other 
See Pilsbry, Guide to Helices, p. XV. 
