﻿SNAIL HOST OF ORIENTAL LUNG FLUKE — ABBOTT 



107 



Two histograms (fig. 44) are presented, with the upper one repre- 

 senting the distribution of individuals based on the measurements of 

 total length of shell, and with the lower histogram representing the 

 same individuals in which the height or length of the last whorl has 

 been measured. Therefore, the upper histogram carries the effect of 

 erosion, and the lower one is without it. Numerous small individuals 

 were not worn away at their spires. These were measured for width 

 and length to ascertain the ratio between these two figures (i. e., the 

 obesity of the shell) . The results are shown in table 5. 



Table 5. — Measurements of shells unaffected by erosion 



In a population sample, the distribution of which approaches the 

 normal curve, or which consists of a uniform sample of adults, it 

 would be necessary merely to multiply the mean of the last whorl 

 measurements by 1.51 (total length/length of last whorl in perfect 

 specimens) to arrive at a theoretical mean of the total shell length. 

 Having obtained this figure, which would represent a population size 

 unaffected by erosion, we need only to compare it with our actual 

 measurements of eroded specimens to ascertain the amount of 

 reduction. 



This direct conversion could not be applied in this case, however, 

 for the histogram is strikingly bimodal and in its lower peak possesses 

 a number of young that were not worn away at the spires. The entire 

 sample, therefore, was arbitrarily divided into groups A and B. As 

 natural a midpoint as possible was chosen between the two peaks in 

 the lower histogram at 6.75 mm. If the erosion-correction factor is 

 applied to this (1.51 x 6.75), we obtain a theoretical equivalent total 

 shell length of 10.19. It so happens that this figure divides the two 

 groups in the upper histogram in exactly the same numerical propor- 

 tion (i. e., 114 in group A and 142 group B) as we have in the lower 

 histogram. 



