The female reproductive system of the sea spider Phoxichilidium femoratum (Rathke, 1799).

Sea spiders (Pycnogonida) are marine chelicerates. Current pycnogonid phylogeny based on molecular data remains uncertain and contradicts traditional morphological perspectives. To resolve this conflict, understanding their inner anatomy is crucial. The reproductive system of sea spiders shows promise as a source of phylogenetic signal, yet our knowledge in this area is limited. This study presents the first description of the whole female reproductive system of a sea spider at the ultrastructural level. We suggest a more detailed functional regionalization of the ovary based on the ovarian wall ultrastructure and distribution of oocyte developmental stages. Meiosis begins in the germarium, and oocytes progress to the vitellarium through a transportational zone. Vitellogenic oocytes extend through the vitellarium wall, connected with it by a stalk - specialized cells. Balbiani bodies are present in early vitellogenic oocytes but dissipate later. The formation of the vitelline envelope, yolk, and fertilization envelope involves functionally diverse RER vesicles. The study also identifies a reproductive sinus as a separate haemocoel compartment that may enhance nutrient concentration near vitellogenic oocytes. Additionally, oviduct and gonopore glands are described in the female of P. femoratum, although their specific functions and prevalence in other sea spider species remain unclear.

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.

Walking leg regeneration in the sea spider Nymphon brevirostre Hodge, 1863 (Pycnogonida).

Regeneration is widespread across all animal taxa, but patterns of its distribution and key factors determining regeneration capabilities stay enigmatic. A comparative approach could shed light on the problem, but its efficacy is limited by the fact that data is only available on a few species from derived taxa. Pycnogonida are nested basally within the Chelicerata. They can shed and replace their walking legs and have a high regeneration capacity. In this work, we carried careful observation on leg appendotomy and regeneration processes in a sea spider under laboratory settings. The limb structure and in vivo observation reveal autotomy as the most likely appendotomy mechanism. High regeneration capabilities were ascertained: an anatomically normal but small leg appeared in a single molting cycle and the full functionality regained in 2-3 cycles. Wound closure after appendotomy in N. brevirostre primarily relies on hemolymph coagulation, which apparently differs from both xiphosurans and crustaceans. Regeneration is provided by proliferation in the leg cutpiece. Regenerative morphogenesis resembles the normal ontogenetic morphogenesis of a walking leg, but accelerated. Unlike in most arthropods, in N. brevirostre, regeneration does not necessarily correspond to the molting cycle, inferring a plesiomorphic state.

Oligomeric larvae of the pycnogonids revisited.

Organization and ultrastructure of the protonymphon larva were never adequately described, despite it being the common larval type of the enigmatic sea spiders and the only example of oligosegmented life stage among recent chelicerates. We have made a comprehensive examination of the newly hatched free-living protonymphons of Nymphon brevirostre using SEM, TEM, light, and confocal microscopy. Although fairly typical in their broad characters, protonymphon larvae have a number of unique and unexpected traits. Body cavity, already present at this stage, is lined with extracellular matrix and thus is conclusively identified as primary body cavity. Central nervous system includes four postocular neuromeres arranged in three ganglia: supraesophageal, subesophageal, and the first ganglion of the ventral nerve cord. Examination of the sensory organs revealed unusually organized eyes, mechanoreceptors, and chemoreceptors. We have uncovered a mixed sensory-secretory nature of chelar glands and proposed possible modalities of its receptory part. We gave first descriptions of the complex ultrastructure of three secretory organs (spinning glands, slit-like organs, proboscis glands) and hypothesized on their mode of functioning. Comparisons with another oligomeric larva, for example, nauplius, revealed discrepancies in the segmentation of these animals. Although both larvae are externally unsegmented and bear three pairs of homologous appendages, the protonymphon body includes a fourth segment of the prospective walking legs which is absent in nauplius.

From egg to "no-body": an overview and revision of developmental pathways in the ancient arthropod lineage Pycnogonida.

BACKGROUND: Arthropod diversity is unparalleled in the animal kingdom. The study of ontogeny is pivotal to understand which developmental processes underlie the incredible morphological disparity of arthropods and thus to eventually unravel evolutionary transformations leading to their success. Work on laboratory model organisms has yielded in-depth data on numerous developmental mechanisms in arthropods. Yet, although the range of studied taxa has increased noticeably since the advent of comparative evolutionary developmental biology (evo-devo), several smaller groups remain understudied. This includes the bizarre Pycnogonida (sea spiders) or "no-bodies", a taxon occupying a crucial phylogenetic position for the interpretation of arthropod development and evolution. RESULTS: Pycnogonid development is variable at familial and generic levels and sometimes even congeneric species exhibit different developmental modes. Here, we summarize the available data since the late 19th century. We clarify and resolve terminological issues persisting in the pycnogonid literature and distinguish five developmental pathways, based on (1) type of the hatching stage, (2) developmental-morphological features during postembryonic development and (3) selected life history characteristics. Based on phylogenetic analyses and the fossil record, we discuss plausible plesiomorphic features of pycnogonid development that allow comparison to other arthropods. These features include (1) a holoblastic, irregular cleavage with equal-sized blastomeres, (2) initiation of gastrulation by a single bottle-shaped cell, (3) the lack of a morphologically distinct germ band during embryogenesis, (4) a parasitic free-living protonymphon larva as hatching stage and (5) a hemianamorphic development during the postlarval and juvenile phases. Further, we propose evolutionary developmental trajectories within crown-group Pycnogonida. CONCLUSIONS: A resurgence of studies on pycnogonid postembryonic development has provided various new insights in the last decades. However, the scarcity of modern-day embryonic data - including the virtual lack of gene expression and functional studies - needs to be addressed in future investigations to strengthen comparisons to other arthropods and arthropod outgroups in the framework of evo-devo. Our review may serve as a basis for an informed choice of target species for such studies, which will not only shed light on chelicerate development and evolution but furthermore hold the potential to contribute important insights into the anamorphic development of the arthropod ancestor.