MANTLE-BODY ARRANGEMENT ALONG THE HINGE OF EARLY PROTREMATOUS BRACHIOPODS - EVIDENCE FROM CROZONORTHIS

The earlier discovery of mantle canals lining the interareas of protre matous brachiopods and the implication that these areas were lined wit h mantle and not simply outer epithelium is supported by evidence from Crozonorthis. In this genus the ventral interareas show a clear exter nal differentiation, reflecting a lining of mantle laterally and of ou ter epithelium medianly. Moreover, this morphology provides clear evid ence, contrary to popular opinion, that setae could develop along the growing margin of a protrematous interarea. A well-defined junction, b etween parts adjacent to the delthyrium which are smooth and lateral p arts with perforations that housed successive generations of setae dur ing life, marks the position where, on the interior, the inner epithel ium separated from the outer epithelium to form the body wall.