The hydrospire system of a spiraculate blastoid probably was two-way: in through pores and out through spiracles, by action of cilia. Pore area increased in size proportionately to increase in volume of hydrospires, which grew throughout ontogeny of the animal. Spiracle area increased at a slower rate, necessitating increasing velocity of output currents with growth.

GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States