Ultrastructure of the lophophoral coelomic lining in the brachiopod Hemithiris psittacea: functional and evolutionary significance

Organization of the blood system of rhynchonellid brachiopod Hemithiris psittacea (Brachiopoda: Rhynchonelliformea)

The brachiopods are sessile invertebrates with an unusual blood system, which consists of a long-branched dorsal vessel. It is still unknown how blood circulates in this system. In the present study, for the first time we propose the circulation of blood in brachiopod Hemithiris psittacea based on morphological and experimental data. The main heart is located on the dorsal side of the stomach and divides the dorsal vessel into anterior and posterior parts. The anterior part enters the lophophore, where it gives off blind branches to each tentacle. The posterior part passes by the funnels of the nephridia and forms a blindly closed network in the gonads. We suggest that the circulation of blood includes three successive stages. During the first phase of systole of the main heart, blood flows through the anterior dorsal vessel. During the second phase of systole, blood flows through the posterior dorsal vessel. During diastole, blood flows from the anterior and posterior vessels and fills the main heart. The origin of a peculiar blood system in brachiopods can be explained by reduction of the ventral vessel, which is probably correlates with the reduction of the ventral side of the brachiopod ancestor's body. Another peculiarity of brachiopod blood system is the presence of an ampullar heart, which functions as a blood depot and allows blood to move in the vessels in two directions in an oscillatory mode. The brachiopod blood system contains vessels lacking true endothelium and can be classified as an "incompletely closed" type.

Structure of the oral tentacles of early ontogeny stage in brachiopod Hemithiris psittacea (Rhynchonelliformea, Rhynchonellida)

Brachiopods have the most complex lophophore in comparison with other lophophorates, i.e., phoronids and bryozoans. However, at early ontogenetic stages, brachiopods have a lophophore of simple morphology, which consists of the oral tentacles. Data on the ultrastructure of the oral tentacles is mostly missing. Nonetheless, it has recently been suggested that the structure of oral tentacles is ancestral for all lophophorates in general, and for brachiopods in particular. The fine structure of the oral tentacles in the brachiopod Hemithiris psittacea is studied using light microscopy, transmission and scanning electron microscopy, cytochemistry and confocal laser scanning microscopy. The oral tentacles have a round shape in transverse section, and four ciliary zones, i.e., one frontal, two lateral, and one abfrontal. Latero-frontal sensory cells occur among the frontal epithelium. Four basiepithelial nerves in the ciliary epithelium are colocalized with ciliary zones. Lophophores of simple morphology in phoronids and brachiopods are characterized by non-specified round forms of tentacles. In phoronids and bryozoans, tentacles have additional latero-frontal ciliary zones that function as a sieve during filtration. In most brachiopods, lateral cilia are involved in the capture of food particles, whereas latero-frontal cells are retained in the frontal zone as sensory elements. The oral tentacles of H. psittacea contain a coelomic canal and have distinct frontal and abfrontal longitudinal muscles, which are separated from each other by peritoneal cells. A similar structure of tentacle muscles occurs in all bryozoans, whereas in phoronids, the frontal and abfrontal tentacle muscles are not separated by peritoneal cells. We suggest that the lophophorates' ancestor had tentacles, which were similar to the tentacles of some phoronids with lophophore of simple morphology. We also assume that the structure of the oral tentacles is ancestral for all brachiopods and the specialization of brachiopod tentacles correlates with the appearance of the double row of tentacles.

Tentacle muscles in brachiopods: Ultrastructure and relation to peculiarities of life style

Although the morphology of the brachiopod tentacle organ, the lophophore, is diverse, the organization of tentacles has traditionally been thought to be similar among brachiopods. We report here, however, that the structure of the tentacle muscles differs among brachiopod species representing three subphyla: Lingula anatina (Linguliformea: Linguloidea), Pelagodiscus atlanticus (Linguliformea: Discinoidea), Novocrania anomala (Craniiformea), and Coptothyris grayi (Rhynchonelliformea). Although the tentacle muscles in all four species are formed by myoepithelial cells with thick myofilaments of different diameters, three types of tentacle organization were detected. The tentacles of the first type occur in P. atlanticus, C. grayi, and in all rhynchonelliforms studied before. These tentacles have a well-developed frontal muscle and a small abfrontal muscle, which may reflect the ancestral organization of tentacles of all brachiopods. This type of tentacle has presumably been modified in other brachiopods due to changes in life style. Tentacles of the second type occur in the burrowing species L. anatina and are characterized by the presence of equally developed smooth frontal and abfrontal muscles. Tentacles of the third type occur in N. anomala and are characterized by the presence of only well-developed frontal muscles; the abfrontal muscles are reduced due to the specific position of tentacles during filtration and to the presence of numerous peritoneal neurites on the abfrontal side of the tentacles. Tentacles of the first type are also present in phoronids and bryozoans, and may be ancestral for all lophophorates.

Detailed morphology of tentacular apparatus and central nervous system in Owenia borealis (Annelida, Oweniidae)

The Oweniidae are marine annelids with many unusual features of organ system, development, morphology, and ultrastructure. Together with magelonids, oweniids have been placed within the Palaeoannelida, a sister group to all remaining annelids. The study of this group may increase our understanding of the early evolution of annelids (including their radiation and diversification). In the current research, the morphology and ulta-anatomy of the head region of Owenia borealis is studied by scanning electron microscopy (SEM), 3D reconstructions, transmission electron microscopy (TEM), and whole-mount immunostaining with confocal laser scanning microscopy. According to SEM, the tentacle apparatus consists of 8-14 branched arms, which are covered by monociliary cells that form a ciliary groove extending along the oral side of the arm base. Each tentacle contains a coelomic cavity with a network of blood capillaries. Monociliary myoepithelial cells of the tentacle coelomic cavity form both the longitudinal and the transverse muscles. The structure of this myoepithelium is intermediate between a simple and pseudo-stratified myoepithelium. Overall, tentacles lack prominent zonality, i.e., co-localization of ciliary zones, neurite bundles, and muscles. This organization, which indicates a non-specialized tentacle crown in O. borealis and other oweniids with tentacles, may be ancestral for annelids. TEM, light, and confocal laser scanning microscopy revealed that the head region contains the anterior nerve center comprising of outer and inner (=circumoral) nerve rings. Both nerve rings are organized as concentrated nerve plexus, which contains perikarya and neurites extending between basal projections of epithelial cells (radial glia). The outer nerve ring gives rise to several thick neurite bundles, which branch and extend along aboral side of each tentacle. Accordingly to their immunoreactivity, both rings of the anterior nerve center could be homologized with the dorsal roots of circumesophageal connectives of the typical annelids. Accordingly to its ultrastructure, the outer nerve ring of O. borealis and so-called brain of other oweniids can not be regarded as a typical brain, i.e. the most anterior ganglion, because it lacks ganglionic structure.

Myoanatomy of the Lophophore in Adult Phoronids and the Evolution of the Phoronid Lophophore

Confocal laser scanning microscopy was used to study the myoanatomy of the lophophore of three phoronids with different types of lophophore: Phoronis ijimai, Phoronis australis, and Phoronopsis harmeri. A four-part ground plan of the lophophoral musculature was detected in all three species and was previously reported for Phoronis ovalis. The ground plan includes (i) a circular muscle, (ii) longitudinal muscles of the tentacular lamina, (iii) groups of paired distal muscles of the tentacular lamina, and (iv) frontal and abfrontal muscles of the tentacles. In P. australis, the tentacular lamina contains strong abfrontal and numerous frontal muscles. Phoronis harmeri has an inner circular muscle and arch-like muscles. Among all studied phoronids, the four-part ground plan of the lophophoral musculature is least complex in P. ijimai, which has a horseshoe-shaped lophophore. The results suggest two possible scenarios by which the morphology of the phoronid lophophore has transformed over evolutionary time. According to the first scenario, the morphology of the ancestral horseshoe-shaped lophophore became more complicated in the case of most phoronids but became simplified in the case of P. ovalis and bryozoans. According to the second scenario, the lophophore gradually transformed from a simple oval shape to a horseshoe shape and then to a spiral shape. The four-part ground plan of the lophophoral musculature is also present in bryozoans, which is consistent with the view that the lophophorates are monophyletic.

Myoanatomy of the phoronid Phoronis ovalis: functional and phylogenetic implications

The myoanatomy of adult phoronids has never been comprehensively studied by fluorescent staining and confocal laser scanning microscopy. Because the organization of the musculature may provide insight into phoronid biology and phylogeny, phoronid myoanatomy warrants detailed investigation. The current study provides the first description based on the use of modern methods of the musculature of the very small phoronid Phoronis ovalis. The musculature of the lophophore base includes radial, longitudinal, and circular muscles; pharynx dilators; and paired lateroabfrontal muscles. The musculature of the anterior part of the body is formed by outer-circular, middle-diagonal, and inner-longitudinal muscles; because all of the cells in these muscles contact the basal lamina, the musculature in the anterior part of the body forms a single layer. In the posterior part of the body, diagonal muscles are absent, and the longitudinal musculature is represented by small, thin bundles. In the terminal end of the body, there is an inversion of circular and longitudinal muscles. The organization of the musculature in the lophophore base and anterior part of the body suggests that the lophophore can move in different directions in order to capture food from local water currents. The organization of the musculature of the terminal end would enable this part of the body to be used for digging into the substratum. The four-partitioned ground plan of the lophophoral musculature in P. ovalis and in bryozoans from all three main groups indicates the homology of the lophophore and the monophyly of the lophophorates as a united clade that includes three phyla: Phoronida, Bryozoa, and Brachiopoda. Some similarities in the organization of the lophophoral musculature, however, may reflect the similarities in the sessile life styles and feeding behaviors of P. ovalis and bryozoans.