Smittoidea propinqua (Smitt, 1868)

Serova, K. M., Belikova, E. V., Kotenko, O. N., Vishnyakov, A. E., Bogdanov, E. A., Zaitseva, O. V., Shunatova, N. N. & Ostrovsky, A. N., 2022, Reduction, rearrangement, fusion, and hypertrophy: evolution of the muscular system in polymorphic zooids of cheilostome Bryozoa, Organisms Diversity & Evolution (New York, N. Y.) 22 (4), pp. 925-964 : 940

Smittoidea propinqua ( Figs. 1D , 10 , 17D )

Avicularium

Cystid and mandible The small suboral adventitious avicularium has an oval outline when viewed from above, with a crossbar ( Fig. 10A ). The avicularian chamber is immersed in the autozooidal frontal shield.

The mandible is hollow, semicircular with small denticles on the distal margin. Its lower wall bears two small depressions. The lower mandibular wall presumably bears an oval opening (visible as a dark area in our CLSM preparations, see e.g. Fig. 10D ) through which the mandible cavity communicates with the avicularian body cavity.

Polypide and muscular system The vestigial polypide is positioned between two large adductors. Few diaphragmatic muscles form a wide “ring” in the uppermost part of the tentacle sheath, which also possesses about 10 longitudinal muscle fibers ( Fig. 10D , inset). The upper ends of the longitudinal muscles intersect with the diaphragmatic muscles. Several thin retractors are attached to the lower part of the tentacle sheath.

Small, smooth, paired abductors are situated close to each other, forming a complete shallow arch of about 10 short bundles consisting of several muscle cells each, and distally attaching to the frontal membrane ( Figs. 10C , 17D ). Their lower wider ends are anchored at the proximal and lateral cystid walls.

Striated adductors, which are the largest muscles of the avicularium, occupy most of the cystid volume ( Figs. 10C, D , 17D ). They insert to the upper vestibular wall, whereas their lower ends are attached to the basal cystid wall.

Figures

Fig. 1 Partial view of living colonies of the studied species (stereomicroscope). A Terminoflustra membranaceotruncata (erect lobate); B Arctonula arctica (encrusting); C Rhamphostomella ovata (encrust-

Fig.10 Morphology and anatomy of adventitious avicularia in Smittoidea propinqua. A Stereomicroscope; B histological sectioning, light microscopy; C, inset, D CLSM, phalloidin staining. A Part of living colony showing autozooids with ovicells (empty and with embryos) and suboral avicularia (some shown by arrows); B Tangential section of avicularium (proximal corner of mandible is seen in

Fig. 17 Schemes of cheilostome autozooid (A), interzooidal avicularium of Terminoflustra membranaceotruncata (B), and adventitious sessile avicularia of Arctonula arctica (C) and Smittoidea propinqua (D). All with uplifted operculum/mandible. Thin muscle fibers radiating from diaphragm shown in B. Longitudinal muscles are visible in tentacle sheath of vestigial polypide in B–D. Abbreviations: db, duplicature bands, dm, diaphragm, f, funiculus, fm, frontal membrane, lw, lower mandibular wall, mab, abductor, mad, adductor, md,