Small, but smart: Fine structure of an avicularium in Dendrobeania fruticosa (Bryozoa: Cheilostomata).

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.