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 accompainying 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 molluscan-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. IA widely held hypothesis concerning the origin of molluscs is the so-called annelid theory. According to Hyman (1951:4), the "annelid theory" states that ". annelids, arthropods, and chordates are closely related and [that] lower bilateral groups may have arisen from annelids by degeneration. " In the present paper the term is restricted, referring to the theory that annelids gave rise to molluscs. Early proponents of this theory (Pelseneer, 1899; Heider, 1914; S6derstr6m, 1925; Naef, 1926) believed that the serial repetition of' certain organs in some molluscs represented vestigial segmentation, and claimed on this basis that molluscs descended from the segmented worms, annelids. They also believed that the farreaching similarities in cleavage and early development were further indications of this relationship. More recent investigators added other viewpoints to these arguments. Thus, Garstang (1928) theorized that molluscs evolved from the trochophore larvae of the annelids by neoteny. Johansson (1952) aimed at giving a functional explanation to the annelid theory, when he proposed that the molluscs had lost the segmentation of their annelid ancestors in turning to living on hard, rocky surfaces of the intertidal zone, and assuming a creeping, crawling mode of locomotion. And finally, Lemche (1957) reported the discovery of a "living fossil," a primitive, segmented mollusc, Neopilina galatheae Lemche, 1957. Soon two more species of Neopilina were found, N. ewingi Clarke and Menzies, 1959, and N. veleronis Menzies and Layton, 1963. These discoveries were accepted by many as evidence for the segmented origin of molluscs (Lemche, 1959a, 1959b, 1960; Lemche and Wingstrand, 1959b; Portman, 1960; Fretter and Graham, 1962). Lemche went so far as to postulate that molluscs are more closely related to the arthropods than to the annelids, and that the latter arose independently of the former two groups. Fretter and Graham supported this argument by claiming that the coelom is primarily small in the arthropods as well as in the molluscs. Opponents of the annelid theory have repeatedly pointed out the weaknesses of it. Thus, Nierstrasz (1922), Hammersten and Runnstrom (1925), Ivanov (1928, 1944), Beklemishev (1958a, 1958b), Boettger (1959), Steinbock (1962) and Hunter and Brown (1965) argued that molluscs cannot be considered as annelids with reduced segmentation, because of fundamental anatomical and developmental differences in