Discoporella terminata

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 : 950

Discoporella terminata ( Figs. 2D , 16 )

Vibracularium

Cystid and mandible The elongated cystids of interzooidal vibracularia are immersed between four neighboring autozooids, being in the center of this quartet ( Fig. 16A, B ). The opesia is r-shaped, with a semioval postmandibular area and a narrow, elongated palate. It is on the same level as the frontal surface of the colony.

The seta (modified mandible) is long, straight, and curved near the base. It has an internal cavity (Fig, 16C–E) that presumably communicates with the vibracularian body cavity. The setal base is thickened and has a complex shape.

Polypide and muscular system The vestigial polypide is oval, consisting of the larger distal part (rudimentary lophophore inside the tentacle sheath) and smaller rounded proximal ganglion with an indistinct constriction between them ( Fig. 16D, E ). The polypide retractor muscle (apparently consisting of two muscle fibers) is attached to this narrow zone ( Fig. 16E ). No diaphragmatic or longitudinal muscles of the tentacles sheath were detected in the histological sections.

The main elements of the muscular system are three large bundles: two adductors and a non-paired abductor. The single smooth abductor arises from the proximal cystid wall, attaching to the thickened cuticular plate of the frontal membrane ( Fig. 16D, E ). The striated adductors are positioned asymmetrically in the cystid cavity. Proximally, they are attached to the opposite lateral cystid walls and, distally, to the setal base ( Fig. 16C, D ). The first bundle is attached to the distal surface of the base, and the second bundle to the setal base proximo-laterally.

Figures

Fig.2 Partial view of living colonies of the studied species (stereomicroscope). A Bicellariella ciliata (erect arborescent); B Aquiloniella scabra (erect arborescent); C Caberea ellisii (erect arborescent); D Discoporella terminata (free-living)

Fig. 16 Morphology and anatomy of vibracularia in Discoporella terminata. A, B Stereomicroscope; C–E histological sectioning, light microscopy. A, B Part of living colony showing frontal colony surface with autozooids and vibracularia with setae in various positions. C–E Sagittal sections showing muscular elements and shape of the