Department of Invertebrate Zoology, Saint-Petersburg State University, Saint Petersburg, Russia.
Laboratory of Evolutionary Morphology, Zoological Institute, Russian Academy of Sciences, Saint Petersburg, Russia.
J Morphol. 2022 Feb;283(2):174-206. doi: 10.1002/jmor.21436. Epub 2021 Dec 28.
Bryozoans are small benthic suspension-feeding colonial animals. Among this phylum, there are representatives showing a lesser or greater degree of polymorphism, and the most common type of polymorphic zooids is the avicularium. Here we present a detailed description of the bird's-head shaped avicularium in Dendrobeania fruticosa. The body cavity of the avicularium demonstrates an acoelomate condition: along the cystid walls, there is neither the layer of extracellular matrix toward the epidermis, nor coelomic lining. However, a layer of extracellular matrix and epithelialized cells lie under the epidermis of the tentacle sheath. Probably, such construction helps the tentacle sheath to acquire some rigidity-it is the only region of the body wall without an ectocyst. We did not find typical funicular strands in the avicularium, but there is a delicate mesh composed of stellate cells with thin and long projections, which sometimes isolate the spaces filled with a heterogeneous matrix. The proximal ends of the adductors, abductors, and polypide retractors are attached to the body wall via typical epidermal tendon cells, which possess numerous bundles of tonofilaments. The distal ends of the abductors and adductors attach to the frontal membrane or upper vestibular membrane, respectively. The inner organic layer of the ectocyst in these regions forms large protrusions, from which numerous thin outgrowths branch off. We suggest them to be a functional analogue of apodemes and apodemal filaments in arthropods. "Apodemal" tendon cells have long and thin projections that line the outgrowths of the ectocyst and surround the distal ends of the muscle cells. At these sites, "apodemal" tendon cells possess numerous tonofilaments. The vestigial polypide includes the tentacle sheath, rudimentary lophophore, cerebral ganglion, and polypide retractors. The sensory part of 5HT-positive cells of the frontal membrane is dendrite-shaped and embedded in the inner organic layer of the ectocyst.
苔藓动物是小型底栖悬浮滤食性群体动物。在该门中，有一些代表具有较小或较大程度的多态性，最常见的多态性生殖体类型是羽枝。本文详细描述了 Dendrobeania fruticosa 中的鸟头状羽枝。羽枝的体腔呈现腔肠动物的无体腔特征：沿触手鞘的囊壁，既没有朝向表皮的细胞外基质层，也没有体腔衬里。然而，一层细胞外基质和上皮细胞位于表皮之下。可能这种结构有助于触手鞘获得一定的刚性——它是体壁唯一没有外胚层的区域。我们在羽枝中没有发现典型的索状结构，但有一层由具有细长突起的星状细胞组成的精细网格，这些突起有时会分隔充满异质基质的空间。收肌、展肌和息肉牵引肌的近端附着在体壁上通过典型的表皮腱细胞，这些细胞拥有许多束张力丝。展肌和收肌的远端分别附着在前额膜或上前庭膜上。这些区域的外胚层的内有机层形成大的突起，从这些突起分支出许多细的突起。我们认为它们是节肢动物的关节突和关节突丝的功能类似物。“关节突”腱细胞具有长而细的突起，沿着外胚层的突起排列，并环绕肌肉细胞的远端。在这些部位，“关节突”腱细胞拥有许多张力丝。残余的息肉包括触手鞘、原始的触手冠、脑神经节和息肉牵引肌。5HT 阳性细胞的感觉部分呈树突状，嵌入外胚层的内有机层中。
