Structure and ultrastructure of muscles in the priapulid Halicryptus spinulosus: functional and phylogenetic remarks

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Abstract
The Priapulida phylum is a rather homogeneous small group of marine invertebrates, living buried in sand and mud of cold waters. These worms present great difficulty in determining their systematic position: in the first instance, Priapulida were considered to be pseudo-coelomates, and classified among the Aschelminthes (Hyman, 1951). How ever, later studies (Shapeero, 1961) have pointed out that the body cavity is actually a coelom, and not a pseudo-coelom. Consequently it seems improper to include Priapulida among Aschelminthes, even if their real position still remains undefined. Today most authors tend to consider priapulids as the last relicts of an originally larger group, forming a separate phylum (van der Land, 1970). We can assume some connexions with other minor Protostoma, but not to an extent sufficient to consider a possible group affinity (Grassé, 1959). The general anatomical organization of a priapulid is now well known (van der Land, 1970), but the only detailed microscopical descriptions are those of the genus Priapulus (Mattisson, Nilsson & Fange, 1974) and the genus Tubilucus (Kirsteuer & van der Land, 1970). We have only incomplete data and summary descriptions concerning the other existing genus. Ultrastructural researches are even more scarce; electron microscope studies could clarify the submicroscopical organization of these animals and allow comparisons with other groups. Afzelius & Ferraguti (1978), for instance, have shown that the sperm of Priapulus caudatus present a very primitive pattern, resembling that of the sperm of many other marine invertebrates with external fertilization. The muscular systems are also useful for researches of phylogenetic correlations: it is, in fact, possible to correlate the ultrastructural organization of the muscles with the systematic position of their respective animals (Hanson & Loewy, 1960; Lanzavecchia, 1977). For the muscular systems of priapulids the only pertinent data are those of Mattisson et al. (1974) on Priapulus caudatus. A detailed analysis of the muscles of other species seems therefore necessary to set up a general picture of the situation in Priapulida. In these animals the muscles are organized to form a typical hydraulic system. The hydraulic systems present one of the most topical problems, from both the morphological and functional points of view (Hammond, 1970; Chapman, 1975).