PCOLOQICAL STUDIES AT LOUQH I N E
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b h e l i weight (9)
FIQ. 61. The relation of shell height to cube root of shell weight for Thai8 (Nucella)
ZapiZZUs from Carrigathorna ( 0 ) and from the Rapids ( 0 ) .
I n the course of collecting the T h i s , the force necessary to remove
them from their natural substrate was measured. For this purpose a
small spring clip, attached by string to a spring balance, was fastened
cautiously to the shell, and the tension was slowly increased until the
Thais was pulled off the rock. The Thais was then placed in a polythene
bowl containing sea water, allowed to become attached, and three
measurements were made of the force necessary to detach it from the
polythene, at intervals of ten minutes. The pull necessary to remove
Thais from either rock or basin was very variable, but the figures for
Carrigathorna include a number which are higher than the peak of those
from the Rapids. There is no reason to suppose that the difference between the groups is other than morphological; it is probably due to a
difference in the relative area of the foot.
Measurements of the force required to break the shells have been
made by a pivoted arm pulled down by a spring balance. The Thais