Functional morphology of the muscles in Philodina sp. (Rotifera: Bdelloidea)

Whole-mounts of Philodina sp., a bdelloid rotifer, were stained with fluorescent-labeled phalloidin to visualize the musculature. Several different muscle types were identified including incomplete circular bands, coronal retractors and foot retractors. Based on the position of the larger muscle bands in the body wall, their function during creeping locomotion and tun formation was inferred. Bdelloid creeping begins with the contraction of incomplete circular muscle bands against the hydrostatic pseudocoel, resulting in an anterior elongation of the body. One or more sets of ventral longitudinal muscles then contract bringing the rostrum into contact with the substrate, where it presumably attaches via adhesive glands. Different sets of ventral longitudinal muscles, foot and trunk retractors, function to pull the body forward. These same longitudinal muscle sets are also used in `tun' formation, in which the head and foot are withdrawn into the body. Three sets of longitudinal muscles supply the head region (anterior head segments) and function in withdrawal of the corona and rostrum. Two additional pairs of longitudinal muscles function to retract the anterior trunk segments immediately behind the head, and approximately five sets of longitudinal retractors are involved in the withdrawal of the foot and posterior toes. To achieve a greater understanding of rotifer behavior, it is important to elucidate the structural complexity of body wall muscles in rotifers. The utility of fluorescently-labeled phalloidin for the visualization of these muscles is discussed and placed in the context of rotifer functional morphology.     

Organisation of body musculature in Encentrum mucronatum Wulfert, 1936, Dicranophorus forcipatus (O. F. Müller, 1786) and in the ground pattern of Ploima (Rotifera: Monogononta)

Fluorescence-labelled phalloidin in combination with confocal laser scanning microscopy (cLSM) has been used to reconstruct the body musculature in Encentrum mucronatum and Dicranophorus forcipatus in order to gain insight into the architecture of body musculature in representatives of the hitherto uninvestigated Dicranophoridae.

In both species, a system of outer circular and inner longitudinal muscles has been found. In E. mucronatum, seven circular muscles (musculi circulares I–VII) and six paired longitudinal muscles (musculi longitudinales I–VI) have been identified. In D. forcipatus, eight circular muscles (musculi circulares I–VIII) and nine paired longitudinal muscles (musculi longitudinales I–IX) are present. In both species, some of the longitudinal muscles span the whole specimen, while others are shorter and connect head and trunk or foot and trunk. Differences in shape and extension of the circular muscles in both species are related to differences in structure of the trunk integument.

Surveying the literature on rotifer musculature, muscles identified in this study are homologised across Rotifera and given individual names. Based on the study of E. mucronatum and D. forcipatus and previous studies on other rotifers, a system of musculature in the ground pattern of Ploima comprising at least three circular muscles (pars coronalis, corona sphincter, musculus circumpedalis) and three pairs of longitudinal muscles (musculi longitudinales ventrales, musculi longitudinales dorsales and musculi longitudinales capitum) is suggested.     

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.     

Volume and morphology changes of a bdelloid rotifer species (Macrotrachela quadricornifera) during anhydrobiosis

Following a study on the changes occurring in a bdelloid species (Macrotrachela quadricornifera, Rotifera, Bdelloidea) when entering anhydrobiosis, we investigated the changes in morphology, including weight and volume during the transition from the active hydrated to the dormant anhydrobiotic state by scanning electron microscopy, confocal microscopy and light microscopy. We compared sizes and morphologies of hydrated extended, hydrated contracted and anhydrobiotic specimens. Bdelloid musculature is defined: longitudinal muscles are contracted in the hydrated contracted animal (head and foot are retracted inside the trunk), but appear loose in the anhydrobiotic animal. When anhydrobiotic, M. quadricornifera appears much smaller in size, with a volume reduction of about 60% of the hydrated volume, and its internal organization undergoes remarkable modifications. Internal body cavities, clearly distinguishable in the hydrated extended and contracted specimens, are no longer visible in the anhydrobiotic specimen. Concomitantly, M. quadricornifera loses more than 95% of its weight when anhydrobiotic; this is more than expected from the volume reduction data and could indicate the presence of space-filling molecular species in the dehydrated animal. We estimate that the majority of body mass loss and volume reduction can be ascribed to the water loss from the body cavity during desiccation.     

Study of architectonics of rotifer musculature by the method of fluorescence with use of confocal microscopy

Musculature of two species of rotifers Testudinella patina (Testudinellidae) and Platyias patulus (Brachiomidae) was studied in confocal laser scanning microscope (CLSM) using fluorescent-labeled phalloidin. It includes cutaneous, visceral, and cutaneus-visceral musculature. The common pattern of structure of the cutaneous musculature is represented by postcoronal circular or transverse muscles and connected with them 2–3 pairs of retractors of the trunk, dorsolateral muscles (17-4), two pairs or bundles of lateral retractors of the corona, circular muscles of the foot, and 10-2 retractors of the foot. Visceral musculature includes muscles of the mastax of both kinds. Spiral-like muscle of cloaca of the T. patina and associated with it V-shaped one as well as strong dorsolateral retractors consisting of 6 longitudinal muscle bundles are typical of Testudinellidae only. Three pairs of cutaneus-visceral muscles bind the musculature of mastax with the body surface in T. patina. Differences in localization and thickness of some elements of musculature of these species are determined by morphological peculiarities of structure of the corona, mastax, and foot, as well as by the rotifer body shape.     

Musculature of Seison nebaliae Grube, 1861 and Paraseison annulatus (Claus, 1876) revealed with CLSM: a comparative study of the gnathiferan key taxon Seisonacea (Rotifera)

The somatic muscular systems of two species of Seisonacea (Rotifera), Seison nebaliae and Paraseison annulatus, are described using fluorescently labelled phalloidin in combination with confocal laser scanning microscopy. Their overall muscular arrangement is similar and consists of segmentally organised longitudinal fibres that extend the length of the body and are surrounded by semi-circular (= incomplete) bands. However, differences in the musculature between the two species are present and possibly reflect specific adaptations in feeding strategy and locomotion related to the occupation of individual niches on their host, the leptostracan crustacean N. bipes. For example, S. nebaliae has semi-circular muscles in the head region only, while P. annulatus possesses incomplete circular muscles also in the trunk region; furthermore, there are also differences in the arrangements and number of longitudinal muscles. The muscular systems of all rotifer species examined so far are compared in order to establish the ground pattern of the last putative ancestor as well as to seek for traits of systematic importance. Results from both species corroborate earlier hypotheses on the arrangement of muscles in the putative common ancestor of Rotifera, which suggested an orthogonal arrangement consisting of a series of probably continuous (not segmental) inner longitudinal muscles, surrounded by semi-circular fibres, ventrally opened. However, significant morphological and ecological variations among taxa investigated so far show that a consistent correlation between muscular traits and specific ecological features and/or phylogeny is still far from being clear. Hence, musculature of additional taxa, representing the systematic width and occupying a diverse range of habitats, should be investigated.     

Musculature of Notholca acuminata (Rotifera: Ploima: Brachionidae) revealed by confocal scanning laser microscopy

Abstract. The body-wall and visceral musculature of Notholca acuminata was visualized using phalloidin-linked fluorescent dye under confocal laser scanning microscopy. The body-wall musculature includes dorsal, lateral, and ventral pairs of longitudinally oriented body retractor muscles, two pairs of head retractors, three pairs of incomplete circular muscles, which are modified into dorso-ventral muscles, and a single pair of dorsolateral muscles. The visceral musculature consists of a complex of thick muscles associated with the mastax, as well as several sets of delicate fibers associated with the corona, stomach, gut, and cloaca, including thin longitudinal gut fibers and viscero-cloacal fibers, never before reported in other species of rotifers. The dorsal, lateral, and ventral retractor muscles and the incomplete circular muscles associated with the body wall appear to be apomorphies for the Rotifera. Muscle-revealing staining shows promise for providing additional information on previously unrecognized complexity in rotifer musculature that will be useful in functional morphology and phylogenetic analyses.     

External Morphology and Muscle Arrangement of Brachionus urceolaris, Floscularia ringens, Hexarthra mira and Notommata glyphura (Rotifera, Monogononta)

We studied four monogonont rotifers (Brachionus urceolaris, Floscularia ringens, Hexarthra mira, Notommata glyphura) using two different techniques of microscopy: (1) the presence of filamentous actin was examined using phalloidin-fluorescent labelled specimens and a confocal laser scanning microscope (CLSM); (2) external morphology was investigated using a scanning electron microscope (SEM). B. urceolaris, F. ringens, and N. glyphura showed similar patterns of muscle distribution: a set of longitudinal muscles acting as head and foot retractors, and a set of circular muscles. However, the size and distribution of circular muscles differed among these species. H. mira differed from the other species in that it lacked circular muscles but possessed strong muscles that extended into each arm. The study showed that using both CLSM and SEM provides better resolution of the anatomy and external morphology of rotifers than using one of these techniques alone. This can facilitate better understanding of the complicated anatomy of these animals.     

The Musculature of <Emphasis Type="Italic">Testudinella patina</Emphasis> (Rotifera: Flosculariacea), Revealed with CLSM

The musculature of Testudinella patina was visualized using phalloidin-linked fluorescent dye by confocal laser scanning microscopy. The conspicuous broad retractors appear to be made up of five separate fibers, of which three anchor in the neck region whereas two extend into the corona. Besides the broad retractors, a total of five paired longitudinal retractors are present and all of them extend into the corona. Incomplete circular muscles are found in groups in the neck region and in the medial and posterior parts of the trunk. The foot musculature comprises eight thin ventral foot muscles and six thicker dorsal foot muscles that all extend from the foot basis to the distal part of the foot. At the basis of the foot, each of the dorsal foot muscles anchors on a smaller, S-shaped subterminal foot muscle. The foot musculature furthermore comprises one pair of paraterminal foot muscles that each anchors basally on a subterminal foot muscle, extends into the most proximal part of the foot and attaches on one of the dorsal foot muscles. The visceral musculature is composed of extremely delicate fibers and is restricted to an area around and posterior to the foot opening. The presence of incomplete circular muscles supports that these muscles are a basal trait for Rotifera, whereas the morphology of the broad retractors and foot muscles is much more specialized and may be autapomorphic for Testudinella or alternatively for this genus and its closest relatives. The present results stress that revealing muscles by staining may produce new information from even well-investigated species, and that this information may contribute to a better understanding of functional as well as phylogenetic aspects of rotifer biology.     

Behavior,metamorphosis, and muscular organization of the predatory rotifer Acyclus inquietus (Rotifera,Monogononta)

Abstract. The atrochid rotifer, Acyclus inquietus, is a sedentary predator that lives within the colonies of its prey, the rotifer Sinantherina socialis. After larvae infiltrate and become associated with the colony, they secrete a permanent gelatinous tube and undergo metamorphosis to the adult stage. We followed settlement and metamorphosis using bright-field microscopy to document specific larval behaviors after eclosion, and used epifluorescence and confocal microscopy of phalloidin-labeled specimens to visualize some of the morphological changes that occur during metamorphosis. Upon eclosion, larvae possess paired eyespots and a ciliated corona that functions strictly in locomotion. After leaving the parent's gelatinous tube, larvae eventually settle on unoccupied colonies of S. socialis or on other substrates if colonies are unavailable. Settlement involves a period of gliding among colony members before attachment with the foot and the secretion of a gelatinous tube. After settlement, there is a drastic reconfiguration of the corona that involves loss of the eyespots, loss of the coronal cilia, and the formation of the cup-shaped infundibulum, a deep depression in the anterior of the head that leads to the mouth. The development of the infundibulum involves the expansion of tissues around the mouth and is accompanied by a reorientation of the underlying musculature that supplies the infundibulum and allows its use in prey capture. The arrangement of the muscles in the trunk and foot regions, which contain outer circular (complete and incomplete) and inner longitudinal bands, remains unchanged between ontogenetic stages, and reflects the condition characteristic of other rotifers.     

Musculature in three species of <Emphasis Type="Italic">Proales</Emphasis> (Monogononta,Rotifera) stained with phalloidin-labeled fluorescent dye

The musculature in the rotifer species Proales daphnicola, P. reinhardti and P. fallaciosa was stained with phalloidin-labeled fluorescent dye and compared using confocal laser scanning microscopy. All three species share several homologous muscle systems, but each systems detailed morphology varies among the species both concerning appearance, number and location. The obtained results were compared with data from other rotifers and it was concluded that the muscles pars coronalis and the corona sphincter probably represent conditions in Ploima or Monogononta, while incomplete circular muscles and dorsal and ventral trunk retractors might be part of the eurotatorian ground pattern.     

Muscle formation during embryogenesis of the polychaete Ophryotrocha diadema (Dorvilleidae) – new insights into annelid muscle patterns

Background

The standard textbook information that annelid musculature consists of oligochaete-like outer circular and inner longitudinal muscle-layers has recently been called into question by observations of a variety of complex muscle systems in numerous polychaete taxa. To clarify the ancestral muscle arrangement in this taxon, we compared myogenetic patterns during embryogenesis of Ophryotrocha diadema with available data on oligochaete and polychaete myogenesis. This work addresses the conflicting views on the ground pattern of annelids, and adds to our knowledge of the evolution of lophotrochozoan taxa.

Results

Somatic musculature in Ophryotrocha diadema can be classified into the trunk, prostomial/peristomial, and parapodial muscle complexes. The trunk muscles comprise strong bilateral pairs of distinct dorsal and ventral longitudinal strands. The latter are the first to differentiate during myogenesis. They originate within the peristomium and grow posteriorly through the continuous addition of myocytes. Later, the longitudinal muscles also expand anteriorly and form a complex arrangement of prostomial muscles. Four embryonic parapodia differentiate in an anterior-to-posterior progression, significantly contributing to the somatic musculature. Several diagonal and transverse muscles are present dorsally. Some of the latter are situated external to the longitudinal muscles, which implies they are homologous to the circular muscles of oligochaetes. These circular fibers are only weakly developed, and do not appear to form complete muscle circles.

Conclusion

Comparison of embryonic muscle patterns showed distinct similarities between myogenetic processes in Ophryotrocha diadema and those of oligochaete species, which allows us to relate the diverse adult muscle arrangements of these annelid taxa to each other. These findings provide significant clues for the interpretation of evolutionary changes in annelid musculature.     

Phalloidin-rhodamine preparations of Macrostomum hystricinum marinum (Plathelminthes): morphology and postembryonic development of the musculature

Summary A whole-mount fluorescence technique using rhodamine-labeled phalloidin was used to demonstrate for the first time the whole muscle system of a free-living plathelminth, Macrostomum hystricinum marinum. As expected, the body-wall musculature consisted of circular, longitudinal, and diagonal fibers over the trunk. Also distinct were the musculature of the gut and of the mouth and pharynx (circular, longitudinal, and radial). Dorsoventral fibers where restricted in this species to the head and tail regions. Circular muscle fibers in the body wall were often grouped into bands of up to four parallel strands. Surprisingly, diagonal fibers formed two distinct sets, one dorsal and one ventral. Certain diagonal muscle fibers entered the wall of the mouth and were continuous with some longitudinal muscles of the pharynx. Dorsoventral fibers in the rostrum occurred partly in regularly spaced pairs, a fact not known for free-living Plathelminthes. All muscle fibers appeared to be mononucleated. During postembryonic development, the number of circular muscle fibers can be estimated to increase by a factor of 3.5 and that of longitudinal muscles by a factor of 2. Apparently as many as 700–800 circular muscle cells must be added in the region of the gut alone during postembryonic development. Stem cells (neoblasts), identified by TEM in the caudalmost region of the gut, lie along the lateral nerve cords. In the same body region most perikarya of circular muscle cells occurred in a similar position. This suggests that the nucleus-containing part of the cell remains in the position where differentiation starts.     

Musculature in sipunculan worms: ontogeny and ancestral states

SUMMARY Molecular phylogenetics suggests that the Sipuncula fall into the Annelida, although they are morphologically very distinct and lack segmentation. To understand the evolutionary transformations from the annelid to the sipunculan body plan, it is important to reconstruct the ancestral states within the respective clades at all life history stages. Here we reconstruct the ancestral states for the head/introvert retractor muscles and the body wall musculature in the Sipuncula using Bayesian statistics. In addition, we describe the ontogenetic transformations of the two muscle systems in four sipunculan species with different developmental modes, using F-actin staining with fluorescent-labeled phalloidin in conjunction with confocal laser scanning microscopy. All four species, which have smooth body wall musculature and less than the full set of four introvert retractor muscles as adults, go through developmental stages with four retractor muscles that are eventually reduced to a lower number in the adult. The circular and sometimes the longitudinal body wall musculature are split into bands that later transform into a smooth sheath. Our ancestral state reconstructions suggest with nearly 100% probability that the ancestral sipunculan had four introvert retractor muscles, longitudinal body wall musculature in bands and circular body wall musculature arranged as a smooth sheath. Species with crawling larvae have more strongly developed body wall musculature than those with swimming larvae. To interpret our findings in the context of annelid evolution, a more solid phylogenetic framework is needed for the entire group and more data on ontogenetic transformations of annelid musculature are desirable.     

The musculature of Squatinella rostrum (Milne, 1886) (Rotifera: Lepadellidae) as revealed by confocal laser scanning microscopy with additional new data on its trophi and overall morphology

Wilts, E.F., Wulfken, D., Ahlrichs, W.H. and Martínez Arbizu, P. 2012. The musculature of Squatinella rostrum (Milne, 1886) (Rotifera: Lepadellidae) as revealed by confocal laser scanning microscopy with additional new data on its trophi and overall morphology.—Acta Zoologica (Stockholm) 93 : 14–27. The monogonont rotifer Squatinella rostrum was investigated with light, scanning electron and confocal laser scanning microscopy to reveal new morphological data on its inner and outer anatomy. In total, the visualized somatic musculature displays five paired longitudinal muscles (musculi longitudinales I–V) and nine circular muscles (musculi circulares I–IX). Compared to other species, S. rostrum is characterized by the absence of several longitudinal and circular muscles (e.g. musculus longitudinalis capitis, corona sphincter and pars coronalis). A reconstruction of the mastax musculature revealed a total number of seven paired and two unpaired mastax muscles. Possibly homologous somatic and mastax muscles in other, thus far investigated rotifers are discussed. Moreover, we provide a phylogenetic evaluation of the revealed morphological characters and suggest possible autapomorphic characters supporting Squatinella and Lepadellidae. Finally, we refer to some striking similarities in the morphology, ecology and way of movement of Squatinella and Bryceella that may indicate a closer relationship of both taxa.     

Macrotrachela quadricornifera (Rotifera, Bdelloidea); a SEM Study on Active and Cryptobiotic Animals

The general morphology of Macrotrachela quadricornifera Milne, 1886 in the active and cryptobiotic states is analyzed under the scanning electron microscope. Some foot peculiarities not previously observed are reported, in particular the canaliculated spurs and presence of a sort of groove on the toes. The integument of active animals is perforated with pores and is micro-sculptured in the trunk region. During cryptobiosis, the animal is contracted, its toot and head being retracted into the trunk: the same microsculpture of the trunk integument is observed.     

A Mitogenomic Re-Evaluation of the Bdelloid Phylogeny and Relationships among the Syndermata

Molecular and morphological data regarding the relationships among the three classes of Rotifera (Bdelloidea, Seisonidea, and Monogononta) and the phylum Acanthocephala are inconclusive. In particular, Bdelloidea lacks molecular-based phylogenetic appraisal. I obtained coding sequences from the mitochondrial genomes of twelve bdelloids and two monogononts to explore the molecular phylogeny of Bdelloidea and provide insight into the relationships among lineages of Syndermata (Rotifera + Acanthocephala). With additional sequences taken from previously published mitochondrial genomes, the total dataset included nine species of bdelloids, three species of monogononts, and two species of acanthocephalans. A supermatrix of these 10-12 mitochondrial proteins consistently recovered a bdelloid phylogeny that questions the validity of a generally accepted classification scheme despite different methods of inference and various parameter adjustments. Specifically, results showed that neither the family Philodinidae nor the order Philodinida are monophyletic as currently defined. The application of a similar analytical strategy to assess syndermate relationships recovered either a tree with Bdelloidea and Monogononta as sister taxa (Eurotatoria) or Bdelloidea and Acanthocephala as sister taxa (Lemniscea). Both outgroup choice and method of inference affected the topological outcome emphasizing the need for sequences from more closely related outgroups and more sophisticated methods of analysis that can account for the complexity of the data.     

Fast drum strokes: Novel and convergent features of sonic muscle ultrastructure,innervation, and motor neuron organization in the pyramid butterflyfish (hemitaurichthys polylepis)

Sound production that is mediated by intrinsic or extrinsic swim bladder musculature has evolved multiple times in teleost fishes. Sonic muscles must contract rapidly and synchronously to compress the gas‐filled bladder with sufficient velocity to produce sound. Muscle modifications that may promote rapid contraction include small fiber diameter, elaborate sarcoplasmic reticulum (SR), triads at the A–I boundary, and cores of sarcoplasm. The diversity of innervation patterns indicate that sonic muscles have independently evolved from different trunk muscle precursors. The analysis of sonic motor pathways in distantly related fishes is required to determine the relationships between sonic muscle evolution and function in acoustic signaling. We examined the ultrastructure of sonic and adjacent hypaxial muscle fibers and the distribution of sonic motor neurons in the coral reef Pyramid Butterflyfish (Chaetodontidae: Hemitaurichthys polylepis) that produces sound by contraction of extrinsic sonic muscles near the anterior swim bladder. Relative to adjacent hypaxial fibers, sonic muscle fibers were sparsely arranged among the endomysium, smaller in cross‐section, had longer sarcomeres, a more elaborate SR, wider t‐tubules, and more radially arranged myofibrils. Both sonic and non‐sonic muscle fibers possessed triads at the Z‐line, lacked sarcoplasmic cores, and had mitochondria among the myofibrils and concentrated within the peripheral sarcoplasm. Sonic muscles of this derived eutelost possess features convergent with other distant vocal taxa (other euteleosts and non‐euteleosts): small fiber diameter, a well‐developed SR, and radial myofibrils. In contrast with some sonic fishes, however, Pyramid Butterflyfish sonic muscles lack sarcoplasmic cores and A–I triads. Retrograde nerve label experiments show that sonic muscle is innervated by central and ventrolateral motor neurons associated with spinal nerves 1–3. This restricted distribution of sonic motor neurons in the spinal cord differs from many euteleosts and likely reflects the embryological origin of sonic muscles from hypaxial trunk precursors rather than occipital somites. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Phenomenon of obligate parthenogenesis in rotifer (Rotifera, Bdelloida)

A comparative analysis of biological and morphological organization of obligate parthenogenous bdelloids and heterogonic monogononts clarifies the specificity of bdelloids' environmental adaptation and variability. It is shown that phenotypic polymorphism of bdelloids differs from that of monogononts. The heterogonic Monogononta are characterized by tremendous phenotypic diversity in body structure adapted to different biotopes in continental waters. This diversity is reflected also in structure of corona and mastax used as major criteria for definition of high range taxa (classes, orders). The polymorphism of bdelloids manifests itself in variability of tiny morphological structures connected with living in restricted volumes of water, mainly in terrestrial biotopes. Being isolated from other Rotifera at the first stages of evolution, bdelloids have been specialized for living under extremely unstable terrestrial conditions that led to the development of anhydrobiosis and loss of amphimixis. Variability of bdelloids is not connected with characters of high taxonomic ranks and has principally different nature in comparison with monogononts. Tempo of specialization seems to be slower in obligate parthenogenous rotifers than in heterogonic ones.     

The fine structure of the drum muscles of the Tigerfish,Therapon jarbua,as compared with the trunk musculature

Summary The fibers of drum and trunk muscles of the Tigerfish, Therapon jarbua, differ greatly in diameter. The myofibrils of the trunk muscles are irregularly oriented, while those of the drum muscles are rolled into spiral or concentric bands. Both muscle types possess the sarcomere structure typical of cross-striated musculature. However, the myofibrils of the drum muscles differ greatly in sarcomere length and width from those in the trunk musculature. The trunk muscles contain few mitochondria, whereas in the drum muscles mitochondria are abundant. The sarcoplasmic reticulum (SR) of the drum muscles takes the form of elongated tubes in both the A and the I region; that of the trunk musculature consists of small vesicles. Of the two muscle types, the drum muscle contains more SR. With respect to the form of the T system, the trunk musculature is of the Z type and the drum muscles of the A-I type. The drum muscle displays a considerably greater number of motor endplates; these lack typical junctional folds and have mitochondria with very few cristae. No fat could be demonstrated in either the drum or the trunk muscles. However, the concentration of glycogen is higher in the drum muscle than in the musculature of the trunk.This work was accomplished with support from the Deutsche Forschungsgemeinschaft and is gratefully dedicated to Prof. R. Danneel on the occasion of his 75th birthday.