The parasitic coelenterate, Polypodium hydriforme USSOV, from the eggs of the American acipenseriform Polyodon spathula.

No significant differences in macro- and micromorphology were found between the parasitic stolon and free-living polyps of Polypodium sp. obtained from infected eggs of the North American acipenseriform fish Polyodon spathula and corresponding developmental stages of Polypodium hydriforme Ussov, parasitic in the Volga sterlet (Acipenser ruthenus). Therefore, both the American and the European forms of Polypodium belong to the species P. hydriforme Ussov.     

Sensitivity of predicted muscle forces to parameters of the optimization-based human leg model revealed by analytical and numerical analyses

There are different opinions in the literature on whether the cost functions: the sum of muscle stresses squared and the sum of muscle stresses cubed, can reasonably predict muscle forces in humans. One potential reason for the discrepancy in the results could be that different authors use different sets of model parameters which could substantially affect forces predicted by optimization-based models. In this study, the sensitivity of the optimal solution obtained by minimizing the above cost functions for a planar three degrees-of-freedom (DOF) model of the leg with nine muscles was investigated analytically for the quadratic function and numerically for the cubic function. Analytical results revealed that, generally, the non-zero optimal force of each muscle depends in a very complex non-linear way on moments at all three joints and moment arms and physiological cross-sectional areas (PCSAs) of all muscles. Deviations of the model parameters (moment arms and PCSAs) from their nominal values within a physiologically feasible range affected not only the magnitude of the forces predicted by both criteria, but also the number of non-zero forces in the optimal solution and the combination of muscles with non-zero predicted forces. Muscle force magnitudes calculated by both criteria were similar. They could change several times as model parameters changed, whereas patterns of muscle forces were typically not as sensitive. It is concluded that different opinions in the literature about the behavior of optimization-based models can be potentially explained by differences in employed model parameters.     

Musculature of an illoricate predatory rotifer Asplanchnopus multiceps as revealed by phalloidin fluorescence and confocal microscopy

The pattern of muscles in the actively swimming predatory rotifer Asplanchnopus multiceps is revealed by staining with tetramethyl-rhodamine isothiocyanate (TRITC)-labelled phalloidin and confocal scanning laser microscopy (CSLM). The major components of the musculature are: prominent semicircular muscles of the corona; paired lateral, dorsal and ventral retractors in the trunk; a network of six seemingly complete circular muscles and anastomosing longitudinal muscles in the trunk; two short foot retractors, originating from a transverse muscle in the lower third of the trunk. The sphincter of the corona marks the boundary between the head and the trunk. The muscular patterns in rotifers with different lifestyles differ clearly, therefore, the muscular patterns seem to be determined by the mode of locomotion and feeding behaviour.     

Organisation of the nervous system in the Acoela: an immunocytochemical study

In order to broaden the information about the organisation of the nervous system in taxon Acoela, an immunocytochemical study of an undetermined Acoela from Cape Kartesh, Faerlea glomerata, Avagina incola and Paraphanostoma crassum has been performed. Antibodies to 5-HT and the native flatworm neuropeptide GYIRFamide were used. As in earlier studies, the pattern of 5-HT immunoreactivity revealed an anterior structure composed mainly of commissures, a so-called commissural brain. Three types of brain shapes were observed. No regular orthogon was visualised. GYIRFamide immunoreactive cell clusters were observed peripherally to the 5-HT immunoreactive commissural brain. Staining with anti-GYIRFamide revealed more nerve processes than did staining with anti-FMRFamide. As no synapomorphies were found in the organisation of the nervous system of the Acoela and that of the Platyhelminthes, the results support the view that the Acoela is not a member of the Platyhelminthes.     

The nervous and muscular systems in the free-living flatworm Castrella truncata (Rhabdocoela): an immunocytochemical and phalloidin fluorescence study

In order to broaden the information about the organisation of the nervous system in the Plathelminthes, an immunocytochemical (ICC) study of the free-living flatworm Castrella truncata (Dalyellioida) has been performed. This is the first time a representative of the taxon Rhabdocoela has been studied with the ICC technique. Antibodies to 5-HT and FMRF-amide and confocal scanning laser microscope were used. 5-HT and FMRF-amide immunoreactivity was observed in the neuropile of the brain, in the cerebral neurones, in the three pairs of longitudinal nerve cords and the adjoining neurones, in the pharyngeal nerve ring and in twelve neurones associated with the pharynx. 5-HT and FMRF-amide immunoreactivity occurs in separate sets of neurones. Only FMRF-amide immunoreactivity was observed in a peculiar ellipse-shaped structure in the brain, and in the genital plexus. The type of orthogon is discussed. Staining of the muscular system with TRITC conjugated phalloidin revealed muscle patterns that have not been described previously.     

Hierarchical genetic algorithm versus static optimization-investigation of elbow flexion and extension movements

The applicability of static optimization (and, respectively, frequently used objective functions) for prediction of individual muscle forces for dynamic conditions has often been discussed. Some of the problems are whether time-independent objective functions are suitable, and how to incorporate muscle physiology in models. The present paper deals with a twofold task: (1) implementation of hierarchical genetic algorithm (HGA) based on the properties of the motor units (MUs) twitches, and using multi-objective, time-dependent optimization functions; and (2) comparison of the results of the HGA application with those obtained through static optimization with a criterion "minimum of a weighted sum of the muscle forces raised to the power of n". HGA and its software implementation are presented. The moments of neural stimulation of all MUs are design variables coding the problem in the terms of HGA. The main idea is in using genetic operations to find these moments, so that the sum of MUs twitches satisfies the imposed goals (required joint moments, minimal sum of muscle forces, etc.). Elbow flexion and extension movements with different velocities are considered as proper illustration. It is supposed that they are performed by two extensor muscles and three flexor muscles. The results show that HGA is a suitable means for precise investigation of motor control. Many experimentally observed phenomena (such as antagonistic co-contraction, three-phasic behavior of the muscles during fast movements) can find their explanation by the properties of the MUs twitches. Static optimization is also able to predict three-phasic behavior and could be used as practicable and computationally inexpensive method for total estimation of the muscle forces.     

Ultrastructure and tubulin immunocytochemistry of the copulatory stylet-like structure in Childia species (Acoela)

One of the main characters used in acoel taxonomy is the male copulatory organ. Despite this, ultrastructural studies of this structure are scarce. We studied the ultrastructure of the copulatory organ in eight species of acoels belonging to the taxon Childia. Members of Childia possess a well-developed conical or cylindrical stylet-like structure composed of needles. Immunogold cytochemistry of tubulin was used to determine the composition of the needles. Stylet-like structures of Childia species at the ultrastructural level are basically similar. Stylet needles show intracellular differentiations. As shown both by ultrastructural and immunocytochemical methods, the stylet needles, in all species studied, are composed of long, parallel microtubules, either tightly packed or polymerized. We report unusual polymerization of microtubules, resulting in formation of a honeycomb-like structure in cross section. Variations of ultrastructure among Childia species include numbers and arrangement of stylet needles, shape of needles, needle compactness, microtubule polymerization, direction of stylet growth, and presence/absence of different types of granules. The stylet-like structures are homologous within Childia, but are likely to prove nonhomologous with the other needle-like structures found in acoel copulatory organs. Stylets in Platyhelminthes are not homologous with stylet-like structures in acoels.     

Ultrastructural and immunocytochemical investigation of acoel sperms with 9 + 1 axoneme structure: new sperm characters for unraveling phylogeny in Acoela

Acoel sperm characters proved useful in deciphering acoel taxonomy. The phylogenetic value of sperm characters in closely related sub-groups or in a monophyletic taxon has not yet been assessed. We have investigated sperm ultrastructure in seven members of the monophyletic taxon Childia sensu (Tekle et al. J Zool Sys Evol Res 43(1):72–90, 2005) and in their closest relatives, the Mecynostomidae (four taxa). All members of Childia examined show little variation in their sperm ultrastructure. The common characters of Childia taxa are: 9 + 1 axoneme structure, the presence of six distal cytoplasmic microtubules in the absence of axial or cortical ones, long nucleus and extensive nucleus–flagella overlap. We have identified a new set of cytoplasmic microtubules lying in the centriolar end of the sperm cell, distal microtubules. The origin and phylogenetic significance of this character is discussed. The types and arrangement of cytoplasmic granules could be used as phylogenetic characters at a low taxonomic level. A loose membrane amorphous core type of granule was found to be a synapomorphy for the following clade within the taxon Childia: C. crassum + C. groenlandica + C. vivipara + C. brachyposthium + C. macroposthium. Sausage shaped granules are plesiomorphic among the taxa examined. The rest of the granule characters were found to be homoplasious. Sperm ultrastructural characters have again proven their concordance with molecular phylogeny. The only morphological synapomorphies known for the sister taxa Childia–Mecynostomidae, in the molecular phylogeny, are characters derived from sperm ultrastructure: distal microtubules arranged in two groups of three microtubules each and a 9 + 1 axoneme structure. The spermatozoa of Childia and Mecynostomidae show 9 + 1 axoneme configuration, seemingly similar to the 9 + ‘1’ axoneme pattern of the Platyhelminthes—Trepaxonemata. Using electron-microscope immunocytochemistry, we have demonstrated that, unlike the central cylinder of trepaxonematans, the central cylinder of the 9 + 1 axonemal pattern in acoels is immunoreactive to tubulin and contains a single central microtubule. Therefore, the 9 + 1 patterns in acoels and trepaxonematans are homoplasious.     

Experimental and modelling investigation of learning a fast elbow flexion in the horizontal plane

Changes in the kinematic and electromyographic characteristics that occur while learning to move as fast as possible have been studied experimentally. Experimental investigation of what happens to the individual motor units (MUs) is more difficult. Access to each MU is impossible, and the recruitment and force developing properties of all individual MUs cannot be known. Thus, what is currently known about MU firing is based on experiments that have recorded relatively few MUs compared to what exists in the entire muscle. A recently developed muscle model (Raikova and Aladjov, 2002, J. Biomechanics, 35, 1123-1135) composed of MUs with different properties can be used for such investigation. The process of learning fast elbow flexion in the horizontal plane was simulated and the results were compared with experimentally measured data. Comparing the simulation results of the very first trial of a particular subject with those of the last trail (at the end of the learning process), it can be concluded that the speed of limb motion and muscle forces increase initially as a result of the more synchronous MUs activation and the increase of firing rate of active MUs. Further improvement necessitated an appreciable reduction in the motor task requirements (i.e. less muscle force and less MUs' activity) set in the computational algorithm by optimization criteria. This forced the next process-inclusion of additional MUs.