Growth and Form of Three Species of Pentremites (Echinodermata: blastoidea)

Waters, Laura

Three species of Pentremites, P. godoni (Defrance) Hall, P. pyriformis Say, and P. robustus Lyon, were studied to determine changes in thecal morphology during ontogeny. These morphological changes were probably related to changing foodgathering requirements during growth. Pentremites godoni and Pentremites robustus show an allometric growth pattern, accompanied by a significant ontogenetic change in thecal form, whereas in Pentremites pyriformis, also demonstrative of allometric growth, there appears less ontogenetic change in thecal form. Increases in ambulacrallength and width, relative to increases in thecal volume, are inferred to enhance food-gathering and respiratory functions of the organism. Although all three species show similar ontogenetic patterns, the increasing thecal volume of P. robustus relative to increases in ambulacral length is greater than that for either P. godoni or P. pyriformis. P. robustus was apparently approaching the maximum size a Pentremites could attain and still maintain a favorable area/volume ratio.