Functional design and evolution of the polychaete Aphrodite aculeata

Abstract

With 10 figures in the text)The body form of Aphrodiie is attributed to its exploitation of the nereid creeping mechanism. It employs a fast stepping pattern, individual limb movements being extension and retraction of the neuropodiuni with co-ordinated chaetal movement. The downward and backward propulsive stroke raises the worm clear of a hard substratum. The notopodium is stabilized and its long chaetae raised and lowered in defence.The segments are short but wide. The body wall muscles, relieved of locomotory function, are a basketwork of longitudinal and diagonal muscles capable of maintaining constant body dimensions. Muscles of the appendages originate from stabilized points on the body wall. Trunk septa are reduced, in association with the constant body shape, but the first few septa form a membrane enclosing the proboscis. Complete and muscular septa occur in the rectal segments.During respiratory movements the elytra are depressed by intrinsic and extrinsic elytrophore muscles and elevated by coelomic pressure. Coelomic pressure drops immediately before elytral depression and is controlled by the body wall muscles, especially the diagonal muscles.Aphrodite differs from its near relative Hermione in the reduction of septa and simplification of body musculature.

Myoanatomy of Myzostoma cirriferum (Annelida, Myzostomida): Implications for the evolution of the myzostomid body plan

“…For example, transverse fibers are present in the dorsal body region in representatives of Hesionidae and Nereididae, whereas they occur in the ventral body region in the Phyllodocidae species (Storch,1968; Tzetlin and Filippova,2005). An additional set of muscles, the so‐called bracing muscles, has been reported from Phyllodocida (Mettam,1971; Tzetlin and Filippova,2005). However, this type of musculature clearly differs from the transverse musculature in that its fibers are arranged in a lattice‐like fashion rather than perpendicularly (Mettam,1971).…”

“…An additional set of muscles, the so‐called bracing muscles, has been reported from Phyllodocida (Mettam,1971; Tzetlin and Filippova,2005). However, this type of musculature clearly differs from the transverse musculature in that its fibers are arranged in a lattice‐like fashion rather than perpendicularly (Mettam,1971). Furthermore, M. cirriferum exhibits an introvert comprising a complete circular musculature.…”

“…Apart from the bilayer of muscle fibers in the body wall, few annelids also show dorsoventral bundles of muscles linking the dorsal and ventral body regions (Tzetlin and Filippova,2005; Purschke and Müller,2006). In addition, polychaetous annelids with parapodia display specific musculature involved in parapodial locomotion (Brown,1938; Dorsett,1961; Mettam,1967, 1971; Storch,1968; Tzetlin and Filippova,2005). However, it is unclear whether the parapodia are an ancestral feature of annelids or whether they have evolved within an annelid subgroup (Westheide,1997; Struck et al,2011).…”

Musculature in polychaetes: comparison of Myrianida prolifera (Syllidae) and Sphaerodoropsis sp. (Sphaerodoridae)

“…For Nereis diversicolor O.F. M üller , 1776 and Aphrodita aculeata L inné , 1758, Mettam (1967, 1971) demonstrated that the “circular portions” of the body wall are transverse fibers belonging to the parapodial complex. In Dorvillea bermudensis A kesson & R ice , 1992 and Dorvillea diadema A kesson , 1974, complete rings of transverse muscles were observed in the zone of segment proliferation, which later on are disrupted by formation of the parapodia and its musculature (M.C.M.…”

Burrowing by small polychaetes – mechanics, behavior, and muscle structure ofCapitellasp.

“…(Seaver et al, 2005). But polychaete musculature has been primarily investigated using dissection and histology methods (Mettam, 1971;Purschke and Müller, 2006;Storch, 1968;Tzetlin et al, 2002a,b) that have some limitations in finding weakly developed muscles . Moreover, most tissue studies use cross-sections or sagittal sections in which helical muscles are not evident without elaborate reconstruction over multiple sections.…”