• Allometry;
  • Blastoidea;
  • hydrospires;
  • Mississippian;
  • ontogeny;
  • Pentremites;
  • respiration

The surface area of blastoid respiratory structures (hydrospires) shows positive allometry during ontogeny to offset the exponential increase in volume. Transverse cross-sections of thecae through an ontogenetic series in two blastoid species, Pentremites pyriformis and Pentremites godoni, were used to calculate surface area and volume within the hydrospires. These two congeneric species showed similar allometric change in hydrospire surface area, but this change was accomplished using different mechanisms. In P. godoni, increased hydrospire surface area was developed through an allometric increase in hydrospire length while keeping hydrospire fold count constant. In contrast, P. pyriformis, showed little allometric change in hydrospire length, but instead increased the number of hydrospire folds ontogenetically. This study shows that developmental patterns can be modified in different ways in order to solve the same functional problem.