The general morphology of Stephanella is similar to other phylactolaemate species (see ); nevertheless, several fine details, particularly of the lophophore structure, are strikingly different from those of any other species.
Epidermis and vestibular wall
The epidermal structure has seldom been used for comparative analyses. Instead, the glandular morphology of the typical gelatinous families lophopodids, cristatellids and pectinatellids was previously recognized as important for its systematic implications . In addition to specific glandular patches, the so-called white spots, that are restricted to a small number of species , two different gland types are generally distinguished: the vacuolar type and the alveolar type. The former are epithelial cells containing a single large vacuole, whereas the latter have numerous smaller vesicles throughout the cytoplasm. Vacuolar cells are also present in Stephanella hina, whereas distinct alveolar cells could not be identified in the current study. In addition to large vacuolar inclusions, smaller translucent vesicular structures were found in S. hina, but these appeared to also occur within vacuolar gland cells. In general, the high heterogeneity of the endocyst structure in S. hina remains poorly understood. It is possible that the presence and thickness of the ectocyst tube is reflected in the glandular composition of the epidermis, especially since the ectocyst is, for the most part, not connected to the epidermis and can also be removed without greater harm to the zooid . Since the ectocyst structure is a diagnostic character for genus classification in plumatellids, the largest taxon of Phylactolaemata, the epidermal structure appears promising for future comparative analyses over the entire range of families and within genera of a specific family, especially in regard to ultrastructure, which was analysed in only a few studies [21, 22] and has never been the specific focus of any.
Likewise, the vestibular wall structure also remains poorly studied (see [4, 19]). It is always a prominent, thickened epithelium as encountered in S. hina. With the exception of Cristatella mucedo, the integration of glandular cells into the vestibular wall has not been described . Stephanella hina shows characteristic, large vacuolar cells (this study), which show little similarity to the glandular cells of C. mucedo. In addition, positive anti-acetylated alpha tubulin staining has never been described in this structure for any other phylactolaemate (see ICC studies on phylactolaemates, e.g., [20, 23,24,25]). The encountered connection with neurite bundles indicates that these cells might be neurosecretory in S. hina. In summary, these appear unique, if not autapomorphic, to S. hina.
Phylactolaemates typically possess a horseshoe-shaped lophophore with an epistome at its base protruding over the mouth opening from the anal side [17, 26]. The coelomic system of the phylactolaemate lophophore shows three distinct canals: a median epistomial canal, which extends from the inner peritoneal lining of the gut shanks into the epistome, the ring canal, which is a short canal supplying the oral tentacles of the lophophore, and the forked canal, which interconnects the cavities of the innermost tentacle row in the lophophoral concavity. Unique to Stephanella hina is the lack of a distinct, continuous forked canal. In all other phylactolaemates, the latter commences on the lateral, proximal sides of the cerebral ganglion as ciliated openings that lead into two ducts that medially fuse in the distal direction. This medial fusion of the ciliated ducts arches directly above the epistomial coelomic extension located above the cerebral ganglion [17, 22, 27, 28]. The few tentacles in the inner lophophoral concavity are supplied by the forked canal. The general layout in S. hina is similar to ciliary patches at the openings of the forked canal ducts, but in S. hina, the coelomic cavities do not fuse at the terminal or median ends.
The epistome structure was most recently studied in several phylactolaemate species [17, 21, 22] and always consists of a highly prismatic epithelium of mostly ciliated cells. To date, distinct holocrine secretion via the epistome epithelium (or the underlying gut epithelium surrounding the mouth opening), as encountered in the current study on S. hina, has not been reported in any other species. Interestingly, the distribution of these secretory processes correlates with the reddish coloration of the same areas in live animals . Colorations or tinges of the epistome are common in phylactolaemates, e.g., with Cristatella mucedo having a brownish hue (and Pectinatella magnifica a bright red coloration in the epistome, e.g., [3, 4]). In such prominent forms with very large zooidal sizes, these colorations are very evident. However, distinct darker hues are also present in other phylactolaemates, such as plumatellids.
Excretion is a little studied process in bryozoans and often seems to be associated with coelomocytes . The secretory mechanism found in Stephanella hina in the present study seems to be newly discovered but poorly understood. The high variability of the epistome structure observed in this study indicates that this process is probably influenced by external factors such as (e.g., physical or temperature-induced) stress. Likewise, the coelomocyte abundance in S. hina appears to be stress-related [Schwaha, personal observation].
The digestive tract of S. hina is similar to that of other phylactolaemates [2, 17]. A persistent problem is the terminology used for and homology of the foregut. In the last summary on bryozoan digestive systems, the phylactolaemate pharynx was always considered the first area of the foregut that bears cilia . Consequently, only the short area surrounding the mouth opening should be addressed as the ‘pharynx’ in S. hina and the remaining, particularly vacuolated epithelium as the esophagus until the cardiac valve. However, several studies (including the present study) frequently label the upper vacuolated area the pharynx (see ). This mostly results from the terminology applied to myolaemates, which are characterized by a myoepithelial, suction pharynx also characterized by vacuolated cells [2, 29]. Similar to S. hina, only a short area close to the mouth opening carries cilia, whereas most of the vacuolated area lacks them. Both, however, are myoepithelial and show distinct cross-striated muscle filaments in their lateral linings. Beyond the vacuolated cells, the esophagus of myolaemates is not myoepithelial or vacuolated. Hence, the mixture of different cytological characteristics makes it difficult to truly assign the parts of the gut.
The typical ciliated pharyngeal area of other phylactolaemates is frequently longer than in S. hina , whereas the vacuolated area of the foregut has either been described with basally located nuclei and single large vacuoles  or, as in S. hina, with nuclei in the middle of the cells with vacuoles on the base and distal areas . As shown in the present study, the nuclei are very small and appear not very active, suggesting that the foregut is mere a mechanical transport tube (see also ). In the plumatellid Hyalinella punctata, presumptive sensory cells are also embedded into this area of the gut , but these were not very abundant in S. hina. The remaining gut of S. hina shows no distinct differences from any other phylactolaemate.
Additional studies should comparatively analyse all six major phylactolaemate families regarding the anatomy and cytological specifications of the foregut. This should aid in redefining the terminology in terms of more criteria than just ciliation and also investigate whether morphological details of the gut might have any systematic or phylogenetic value.
The myoanatomy of phylactolaemates has been studied recently with f-actin staining in four of its six major families, including Plumatellidae and Fredericellidae by Schwaha & Wanninger  and Pectinatellidae and Cristatellidae by Gawin et al. . The current investigation on the fifth family, Stephanellidae, with its current sole representative Stephanella hina, shows that the main muscular systems are very similar to previous descriptions. Concerning the six abovementioned muscular systems, the following can be summarized. 1) The body wall possesses a regular muscular grid as present in other phylactolaemates [2, 4]. A diagonal muscle layer, as found in Pectinatella or lophopodids , was not found. Hence, a regular orthogonal grid is most likely the ancestral condition. 2) The apertural muscles with duplicature bands and vestibular dilatators are in accordance with most previous descriptions of phylactolaemates [15, 17]. Accordingly, since the duplicature bands attach to the distal tentacle sheath in S. hina, the situation is similar to that of most phylactolaemates except lophopodids where the bands attach at the diaphragmatic sphincter . Hence, the more ubiquitous insertion of duplicature bands at the tentacle sheath rather than the diaphragm appears to be the ancestral condition for phylactolaemates. This is also supported in non-phylactolaemates where the bands always insert at the tentacle sheath . The insertion area at the diaphragm in reported lophopodids is probably a derived and apomorphic feature. Most bryozoans, including many phylactolaemates, have a pronounced vestibular wall area, which, however, is little pronounced in S. hina. Hence, the differentiation of the diaphragmatic sphincter structures is not as evident as in other species (see [30, 31]).
3) The tentacle sheath muscles comprise more prominent longitudinal and circular muscles. The arrangement thus reflects the general arrangement of the body wall musculature, as also seen in S. hina. This supports the notion that a regular grid of orthogonal musculature is the ancestral condition for phylactolaemates (see [17, 30, 31]). The condition of plumatellids solely bearing longitudinal muscles in the tentacle sheath is thus probably derived, similar to the remaining bryozoans, which also usually only possess longitudinal muscle fibres in the tentacle sheath [2, 17]. 4) The digestive tract is similar to that in other described phylactolaemates (see [17, 30]) in that it is supported only by circular muscles. No longitudinal muscle fibres were found, as indicated for the lophopodid Asajirella gelatinosa . In contrast to other species, S. hina has smooth muscle fibres in the caecum lining, which is striated in other forms (see above citations). In addition, the arrangement of the muscle bands is very dense in other phylactolaemates, whereas S. hina shows a rather loose pattern, particularly in the caecum. In contrast to all other analysed phylactolaemates, S. hina does not show longitudinal muscles in the funiculus (see ).
5) In all bryozoans, including phylactolaemates, the tentacles are supplied with two longitudinal muscle bands . These are mostly smooth fibres, which is also the case in Stephanella hina. The lophophoral base area where these muscle bands insert differs between phylactolaemates and myolaemates, with a series of small muscular elements being present in the latter. In S. hina, these rootlets are negligible since the frontal muscle bands appear more distally and show no rooting of any sort, and the abfrontal muscles only have a thin elongated proximal extension. This differs from the previous description of the other four families, where the frontal tentacle muscles show one to two different rootlets, which on the oral side are even connected to the pharyngeal musculature. Likewise, the abfrontal muscle bands have a characteristic arrangement not found in S. hina, where only a series of a few stacked muscle bundles, intertwined in a zig-zag fashion, are present. Consequently, S. hina entirely differs in its basal root structure of the tentacle musculature from the other four analysed families [30, 31].
The epistomial musculature can show two different patterns: muscle fibres embedded in the epithelial linings or individual muscle fibres traversing the epistomial coelom/cavity(see [17, 28, 30, 31]), although a mixture of both systems was also detected in the plumatellid Hyalinella punctata . In S. hina, the second state, involving individual fibres, is present and shared by lophopodids and pectinatellids, whereas the three remaining families, Cristatellidae, Fredericellidae and Plumatellidae, share the first configuration. It thus remains difficult to assess which type of epistome musculature is ancestral, but since lophopodids and stephanellids are early branching, it seems reasonable to assume individual traversing fibres as the ancestral condition.
Muscles associated with the proximal border of the lophophoral ring canal were detected in Cristatella mucedo, Pectinatella magnifica and Hyalinella punctata  and were probably overlooked in other plumatellids and fredericellids (Schwaha, personal observation). We could not detect any muscles associated with the ring canal, which indicates that these evolved within phylactolaemates or were lost by Stephanella.
6) The retractors show no difference from those of any studied phylactolaemates (see references above). They consist of numerous bundles originating from the body wall and attach at several locations on the oral side of the polypide. They are of a smooth fibre type.
The general structure of the nervous system is similar to that of other bryozoans [17, 23, 25, 32]. The most striking difference is the unique arrangement of the ganglionic horns in Stephanella hina. These proceed distally from the lateral sides of the cerebral ganglion and bend medially to extend towards the lateral margins of the epistome. In other phylactolaemates, the ganglionic horns extend in a straight line distally towards the tip of the lophophoral arms [23, 27, 31, 33], and see Fig. 17). Since S. hina is the only representative with such an unusual condition, it is concluded that this characteristic (like the general lophophore situation, see above) is apomorphic. The traverse of the bent ganglionic horns is reminiscent of the epistomial nerve (see ), but such a nerve could not be detected in more recent analyses. Instead, the epistome is innervated by a plexus emanating directly from the cerebral ganglion, as also described for Hyalinella punctata :
Additional differences in the nervous system are evident in the detailed tentacle innervation that in Stephanella hina appears to completely lack latero-abfrontal neurite bundles in each tentacle, which are otherwise found in all other phylactolaemates [23, 25] and most other lophophorates . The abfrontal neurite bundle is always formed by the medial fusion of two intertentacular roots. In other phylactolaemates, the lateral roots continue as latero-abfrontal neurite bundles. However, S. hina seems to be the first phylactolaemate recently investigated that lacks any sign of these latero-abfrontal neurite bundles. The current data do not support any abfrontal lateral neurite bundles, but to completely verify the presence or absence of these thin neurites, transmission electron microscopy would be necessary.
In addition to the apparent lack of latero-abfrontal neurite bundles, the roots of the medio-frontal bundles mostly emerge directly from the circum-oral nerve ring (CON), with only a few emerging from the radial nerves. The latter condition is described for most other phylactolaemates [23, 25]. Hence, the condition in S. hina is reminiscent of the medio-frontal nerves of myolaemate bryozoans that show a similar origin with mostly two roots emerging intertentacularly from the CON [16, 17, 35, 36].
Concerning the remaining, more peripheral neuronal elements, the plexus innervating the digestive tract and tentacle sheath is more diffuse and does not show more prominent neurite bundles on either the oral or anal side of the polypide, which was reported for all other species [23, 25, 32, 37]. Concentrated neurite bundles, as found in most phylactolaemates, are also the usual condition in myolaemates that lack a plexus in these areas. The remaining gut shows as little innervation with staining against acetylated alpha-tubulin as previously reported for Hyalinella punctata  and Cristatella mucedo . Distinct intraepithelial, probably sensory, cells, as found in S. hina, were also encountered in the aforementioned species.