[Untitled]

Due to their non-segmented coelom the molluscs occupy a position somewhat away from the centre, or at the margin, of coelomate radiation (Chap. 2). Spiral segmentation of the egg is common to molluscs, annelids, and certain platyhelminthes (Polyclads, Turbellaria, Nemertids), and the trochophore larva of the annelids is also developed in the gastropods, scaphopods and many bivalves. However, the paired organs (muscles) known from fossil monoplacophorans or from the extant genus Neopilina (branchia, nephridia) are not vestiges of a primitive segmented annelid ancestor. Rather, they are independent acquisitions developed from a seriated protocoelomate, a common ancestor to the coelomates, and generally preserved by primitive forms of the phylum. Recent comparative results from molecular biology on the nucleotide sequence of ribosomial RNA again suggest closer ties between the annelids and the molluscs with a common source close to the arthropods.