These changes in coelomic fluid PCO2 parallelled the changes in CO2 production in air. In echinoderms changes in pH are primarily influenced by the PCO2 of the body fluids . Here the increase in PCO2 leads to a decrease in pH by about 0.3 units in 15 and 25 °C. There was no change in pH in 5 °C and likewise there was no significant increase in coelomic fluid PCO2. However, there was probably also a concomitant rise in pH caused by the low temperature. Unlike the intertidal urchins P. miliaris and
E. esculentus there was no compensation for the acidosis. Generally echinoids have a better buffering ability than asteroids because the starfish can exchange CO2 through the bodywallmore efficiently, whereas the hard test of the urchins retards CO2 excretion