Ultrastructure of the Visceral Coelom and Musculature in Novocrania anomala (Brachiopoda, Craniiformea) With Detailed Discussion on the Valve-Opening Mechanism

Craniiformea is a clade of brachiopods, insufficiently studied in terms of functional morphology. While the valve-opening mechanism in Linguliformea and Rhynchonelliformea has been reconstructed, the data on the same mechanism in Craniiformea are incomplete, although several hypotheses concerning this issue have been provided since the end of the 19th century. To review these hypotheses, we have studied the ultrastructure of the main visceral coelomic compartments and the muscles involved in shell movements in Novocrania anomala. Our data document that the lateral oblique muscles, together with a well-developed longitudinal musculature of the body wall (described in craniiforms for the first time) compress the perivisceral coelom along the antero-posterior axis. As a result, the perivisceral coelom expands dorso-ventrally, pushing the dorsal valve up. Thus, that the valve-opening mechanism in Craniiformea is, in principle, similar to that in Linguliformea. We also describe the smooth and cross-striated parts of the anterior adductors, and demonstrate that all muscles of N. anomala are formed by myoepithelial cells.