Reaction of Ciliates to Environmental Factors

Highand low-level Acmiaea limatula differ in the relation between shell and body weight, and in shell volume and the size of the extra-visceral space. High-level shells are thicker and heavier for a given weight of soft parts. Although highlevel shells hold less volume than low-level shells of the same weight, the smaller body more than offsets this. Therefore, high intertidal individuals, living under desiccating conditions, have a larger water space between the shell and soft parts. It is suggested that the greater water-holding capacity of high-level animals is adaptive but only if a larger extra-visceral space means a larger water volume. Given equal area for evaporation (at the junction of the shell and substrate) it must be assumed that either (1) the entire extravisceral space is filled with water, or (2) the proportion of air is constant for a given size space. The percentage water loss would then be less in those animals with the larger extra-visceral space.