The muscles, body wall and valve-opening mechanism of extant craniid (inarticulated) brachiopods

Abstract

Specimens of six species of craniid brachiopod were dissected and the musculature, body wall and mantle were examined. New names are proposed for the "brachial protractor" and "brachial retractor" muscles. The "brachial elevator" muscles are part of the anterior adductor muscles, a small bundle of translucent quick-muscle partially enclosed by a larger, curving bundle of the dark-coloured slow-muscle. A new ring muscle is described. The lophophore arms are extended and retracted by the hydrostatic skeleton and brachial muscles respectively. The valves of Novocrania are probably opened by the creation of hydrostatic pressure, within a newly described coelomic chamber, by contraction of the oblique lateral muscles. Neoancistrocrania has an extra pair of dorsal mantle canals arising at the posterior and a dorsally directed pouch in the body wall.

Brachiopods: origin and early history

“…(). Recent craniiforms lack articulation and open their shells hydraulically with the assistance of the outside lateral muscles attached anteriorly to the body wall (Robinson ). This shell‐opening mechanism differs markedly from that found in the lingulates, which have well developed dermal muscles (Hyman ; Popov et al .…”

Fossil craniid brachiopods (Craniata) of Australia and New Zealand

“…Robinson and Lee (2007, p. 129) suggested that 'by having the brachial protractor muscles attachment point situated more ventrally, on the processes, V. quenstedti may be able to manipulate its lophophore more effectively as the muscles are not all contracting dorsally'. Robinson (2014) found that the so-called brachial protractor muscles do not attach to the lophophore and renamed them the small anterior muscles. The function of these very small muscles, and how this function is (presumably) enhanced by the development of the processes they attach to in Valdiviathyris, remains unknown.…”

“…New terminology for some craniid muscles, based on dissections of specimens of six extant species, was introduced by the author (Robinson 2014). New terms include small anterior muscles (previously known as the brachial protractor muscles, Blochmann [1892]) and support structure (previously known as the brachial retractor muscles).…”

“…New terminology includes organic pads for the organic rostella and muscle attachment surfaces, muscle attachment tissue for the tissue that organic pads are composed of, and latent muscle scars for the shallow to deep depressions and holes through the ventral valve left behind when the muscle attachment tissue pads have decayed away (Robinson 2017b). Robinson (2014) also introduced the term ventral mound for the central raised structure where muscles attach on the ventral valve. However, there is an older term for this structure introduced by Höninghaus (1828), the rostellum, that has priority and the term ventral mound is therefore discarded.…”

Atlas of Antarctic and sub-Antarctic Brachiopoda

“…82°47'O -- 8 Octobre 1898 -Profondeur : 500 m environ." Depth range: 40 --3310 m. References: Brand et al (2003), Cohen et al (2014), Cooper (1973a, Dall (1921), Eichler (1911), Foster (1969Foster ( , 1974, Hatai (1965), Robinson (2014), Sáiz et al (2008), Thomson (1918), Zezina ( , 1981Zezina ( , 1994Zezina ( , 1998, Zezina & Pakhnevich (1998).…”