LYMN^ID^E OF NORTH AMERICA. 13 



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 . Lymnaeas. 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 Lymncea 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 Lymnseas 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 1 . 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 Lymnaea 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 oesophagus, immediately posterior to 

 the buccal sac (plate I, 47). In this nerve ring the two cerebral gan- 

 glia are above the oesophagus 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 



Pilsbry, Guide to Helices, p. XV. 



