On the Origin of Molluscs, The Coelom, and Coelomic Segmentation

Molluscs had a common origin with the annelids, as shown by remarkable ontogenetic similarities. Their common ancestors were non-segmented (non-eumetameric), acoelomate animals. Molluscs developed from the common stock through incorporation of the distinguishing molluscan characters such as the shell, mantle, mantle cavity, radula and ctenidium and annelids by the adoption of coelomic segmentation and of changes accompanying it. The assertion that molluscs hand ancestors with coelomic segmentation is contradicted by ontogenetic and anatomical facts as well as by theoretical considerations. The common moHuscan-annelid stock most likely originated from ancestral flatworms; this is indicated by the developmental, and to a lesser extent, structural similarities between the flatworms, molluscs and annelids, and by the lower level of organization of the flatworms. The coelom is a phylogenetically new structure in the molluscs and annelids, and was acquired independently in the two phyla. Coelomic segmentation also is a phylogenetically new acquisition of the annelids; it came about through developmental changes affecting the growth of the mesoderm bands. These views are the corollaries of ontogenetic considerations.