Abstract
Fauchald, K. (Allan Hancock Foundation, University of Southern California, Los Angeles, California 90007) 1975. Polychaete phylogeny: a problem in protostome evolution. Syst. Zool. 23:493-506.-The evolution of the protostome coelom is an adaptation to burrowing as is segmentation in the annelids. The burrowing ancestral polychaete resembled in body-form recent terrestrial oligochaetes. Recent polychaete families originated in an adaptive radiation following the migration of the ancestral annelids into the flocculent detrital layers in shallow marine muds and from there onto hard bottoms and other specialized environments. [Polychaetes; phylogeny; protostomes.] At present about seventy-five polychaete families are known (Fauchald, in preparation). Most familie's can be defined on morphological characters and recognized by non-specialists once the terminology is mastered. Attempts at grouping the families into acceptable higher taxa have been uniformly unsuccessful. Clark (1969) remarked that most polychaete taxonomists have overlooked all attempts and continued to use two old concepts: sedentary and errant polychaetes, vaguely identified as orders, but not really used as such and only rarely defined (but see Hartmann-Schroder, 1971). The two orders are practical groupings that subdivide the families into two equally large groups. The lack of success in erecting higher taxa can be traced to two distinct complexes of causes. First, the variability of structure within each family has been poorly known (cf. Orrhage, 1966). Second, an interpretation of the phylogeny of any group depends on knowledge of the origin of the group with a consequent distinction between primitive and advanced traits. Interpretation of the polychaete phylogeny is thus dependent on an understanding of the origin of the segmentation and thus of the secondary body-cavities. This implies knowledge of the origin of the mesoderm and ultimately the origin of the metazoans. One has to account for the living condi' Contribution from Santa Catalina Marine Biological Laboratory number 12. tions of the organisms to interprete their phylogeny (Clark, 1964; 1969). Proposed hypothetical animals will have to be capable of performing all tasks of a living organism; they should also be at least as well adapted to their environment as contemporary organisms. A number of morphological studies have demonstrated the limits of variability within several polychaete families (cf. Orrhage, 1964; 1966; Mettam, 1967; 1971 and others). Major synthetic papers have considered both morphological (Storch, 1968) and overall phylogenetic interpretations (Clark, 1969). A very important review of the eversible pharyngeal structures in polychaetes by Dales (1962) started a series of new investigations into these structures (but see Orrhage, 1973b). A modified definition of primitive and advanced features in annelids and a new phylogenetic pattern for the polychaetes is given below. Theories on the origin of the secondary body-cavities and segmentation have been briefly reviewed to clarify the structure of the most primitive annelids. ORIGIN OF COELOM AND SEGMENTATION Three current theories attempt to account for the phylogenesis of the secondary body cavities. These include the gonocoel theory, the enterocoel theory and Clark's hydrostatic theory (cf. Hyman, 1951; 1959; Jagersten, 1955 and Clark, 1964; 1969). The coelomates are assumed to be monophyletic
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