Terminoflustra membranaceotruncata (Smitt, 1868)
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publication ID |
https://doi.org/ 10.1007/s13127-022-00562-y |
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persistent identifier |
https://treatment.plazi.org/id/03AC87AF-FFA4-FFA0-FC98-CA9480B79EDC |
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treatment provided by |
Felipe |
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scientific name |
Terminoflustra membranaceotruncata |
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Terminoflustra membranaceotruncata View in CoL ( Figs. 1A View Fig , 7 View Fig , 17B View Fig )
Avicularium
Cystid and mandible The box-like interzooidal avicularia of T. membranaceotruncata are situated among autozooids, and their basal walls are involved in the formation of the abfrontal surface of lobes in a single-layered colony together with basal walls of autozooids. Nevertheless, avicularia are considerably smaller than feeding zooids ( Fig. 7A View Fig ).
Basal, transverse (proximal and distal) and two lateral walls are weakly calcified, whereas the most part of the (note large opening in lower mandibular wall). Thin fibers radiating from tentacle sheath and diaphragm area are visible in C, E, and F. In F dotted line indicates borders of avicularian cystid. Abbreviations: av, avicularium, bw, basal wall of cystid, c, avicularian cystid, dm, diaphragm, fm, frontal membrane, g, ganglion of vestigial polypide, mab, abductor, mad, adductor, md, avicularian mandible, p, palate, pvm, parieto-vestibular muscles, rm, retractor muscles of vestigial polypide, rt, reduced tentacle of vestigial polypide, sc, sclerite of mandible, t, tendon of adductor, tm, transverse wall of mandible, ts, tentacle sheath, tw, transverse wall of cystid, uv, upper wall of vestibulum, vm, muscles of lower wall of vestibulum, vp, vestigial polypide frontal wall is membranous with a narrow gymnocystal fringe. Both palatal and postmandibular areas have a narrow cryptocyst. In some cases, the endozooidal ovicells formed by the proximal part of the avicularian cystid were recorded. Some of them contained an embryo.
The sclerotinized “squared” mandible with semicircular distal outline occupies somewhat more than half of the avicularian frontal wall ( Figs. 7A, B, F View Fig , 17B View Fig ). The mandible is hollow, consisting of three walls and enclosed cavity lined by a single layer of epidermal cells. An external (upper) and internal (lower) walls merge at the free mandible edge and form a thickened, solid “arch-like” peripheral sclerite, narrow laterally and wider distally ( Fig. 7B–D View Fig ). Proximally, the upper and the lower mandibular walls are connected with each other by a thick vertical wall. The proximal edge of the upper mandibular wall continues to the frontal membrane of the avicularium. The lower wall has a large oval (sometimes asymmetrical) opening ( Figs. 7B, F View Fig , 17B View Fig ) connecting the mandible cavity with a body cavity of the avicularium. Most of the lower wall of the mandible (except sclerite) is internal, having the upper vestibular wall beneath. This wall is distally and laterally attached to the sclerite, and extends proximally towards the mandible base, subsequently continuing to the palate (lower vestibular wall of avicularium bearing orifice) ( Fig. 7C View Fig ). The space between the upper and lower vestibular walls is termed vestibulum, as in the autozooid ( Fig. 7C View Fig ).
Polypide and muscular system The vestigial polypide of the avicularium is situated in the distal part of the cystid near the mandible base. It is reduced to a small oval “organ” consisting of a single knob-like rudimentary lophophore surrounded by the tentacle sheath, and of the basally positioned, rounded cerebral ganglion ( Figs. 7C View Fig , inset, 17B). We recognized some elements of the apertural musculature characteristic for the autozooid, including an annular diaphragmatic sphincter and transverse muscles of the lower vestibular wall ( Fig. 7E, F View Fig ). Two large parieto-vestibular muscle bands arise from the lower part of the avicularian cystid and insert, widely branching, to the vestibular wall ( Fig. 7E View Fig ). The wall of the tentacle sheath bears several thin longitudinal muscle fibers. Finally, several thin muscle fibers radiate from the diaphragm area distally and proximally ( Figs. 7E, F View Fig , 17B View Fig ); similar muscle fibers were also detected in autozooids but their affinity is currently unclear. Four to five retractors insert into the tentacle sheath, anchoring the basal cystid wall ( Fig. 7F View Fig ). Except the diaphragmatic sphincter and retractors, all other aforementioned muscles were recorded in aviculariua for the first time.
The most prominent avicularian muscles are paired abductors and adductors ( Figs. 7C–F View Fig , 17B View Fig ). Abductors operate the opening (uplift) of the mandible by flexion of the frontal membrane. Each smooth abductor comprises 2–3 dense rows of about 20 parallel bundles, situated along the lateral margins of the frontal membrane that occupies the proximal half of avicularium ( Fig. 7D–F View Fig ). Muscle bundles have approximately the same width and length and consist from 2–3 to several fibers (muscle cells) each. Proximally, abductors are attached to the lower part of the lateral cystid walls, then ascend obliquely, and finally insert to the frontal membrane at an acute angle. The attachment zones of the two abductors to the frontal membrane are narrow and parallel to each other ( Fig. 7E, F View Fig ). The distal portions of abductors, when viewed from above, are situated above the proximal parts of adductors.
Closing (lowering) of the mandible is carried out by a contraction of two striated adductors that are the largest muscles of the avicularium ( Fig. 7C–F View Fig ). They consist of multiple muscular cells extending from basal and lateral cystid walls and forming several thicker bundles. Finally, each adductor ends with a tendon ( Fig. 7B View Fig ) that runs between the lower mandibular wall and the upper vestibular wall, being distally inserted to the upper wall of the vestibulum ( Figs. 7B, D View Fig ). The distal ends of both tendons widen at their attachment sites ( Fig. 7B View Fig ). The proximal attachment zones of the adductors occupy most of the avicularian basal wall.
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