Opening and slow closing movements
(Fig. 408–409) of the valves are governed by
fluctuations in pressure within the meta coelomic
body cavity and are generated by
contraction of the lateral muscle layers of the
body, which are composed of circulo-longitudinal fibers. This body cavity functions
as a single, fluid-filled chamber, although
partially divided by a gastric parietal
band and, with the coelomic canal of the
pedicle, acts as a fluid reservoir in the hydraulic
system. This system that opens the
valves performs the same function as the
elastic hinge ligament of the molluscs and
the diductor muscles of articulated brachiopods.
Quick closure is obtained by the contraction
of the anterior and posterior adductor
muscles. Scissor Like movements of the
valves occur by contraction of the well developed,
oblique muscles. This complex
body musculature sustains the unique, infaunal
mode of life of these brachiopods.