﻿102 



PROCEEDINGS OF THE NATIONAL MUSEUM 



NO. SPECIMENS 



100 



STATION 



LENGTH OF SHELL fMM.) 



Figure 42. — Population curves based on shell length in three samples from Lithia Spring. 

 Fla. Circles denote point at which maturity is reached. 



due to a seasonal, environmental stimulus. However, the distance 

 between the "wave" of young and the adults varies considerably from 

 6 mm. in population 7 to mm. in population 3. The latter popula- 

 tion, which is unimodal, may represent a population in which the 

 "wave" of immature has grown into the mature class, and it is not 

 unlikely that it was bimodal a few weeks before the date of collection, 

 in the manner of the other populations. 



The independent but pronounced production of young in the various 

 populations suggests a cyclical activity in the physiology of the 

 population itself, in which a fluctuation of sexual activity is present. 

 How frequently these waves of young are produced could not be 

 ascertained from data gathered on one visit to the spring. 



McMullen (1947), in his studies on the growth rates of the fresh- 

 water snail Oncomelania quadrasi Moellendorff in the Philippine 

 Islands, found similar smaller peaks or waves in the measurements 

 of colonies. By revisiting his collecting areas periodically he was 

 able to watch these waves of young move slowly toward the main 

 peak of the adults. From his data he was able to ascertain that that 

 species reaches maturity in 4 to 5 months. I have shown also (1946) 

 that similar waves occur in the developmental stages of the eggs of 

 Oncomelania^ and that this was due to increased ^g% production 

 brought about by the stimulus of rains and flood periods. 



