Comment on ‘Stable isotopes challenge the perception of ocean sunfish Mola mola as obligate jellyfish predators'

Muscle development in the bamboo sole Heteromycteris japonicus was investigated, focusing primarily on the cranial muscles, using an improved whole mount immunohistochemical staining method with potassium hydroxide, hydrogen peroxide and trypsin. Larvae of H. japonicus had branchial levators, but not all of them were retained in adults, a condition also seen in the Japanese flounder Paralichthys olivaceus. In particular, larval branchial levators II and III disappeared during development, while I and IV remained to become the levator internus I and levator posterior, which were well‐defined muscles in adults. In place of the atrophied muscles, levatores externi and levator internus II developed and regulated the branchial arches. The results showed that the muscle composition in the dorsal branchial arches changed to the adult form before metamorphosis in H. japonicus, as seen in P. olivaceus, and this transformation may be common to all members of that group.     

Cephalic muscle development in the Australian lungfish,Neoceratodus forsteri

Lungfishes are the extant sister group of tetrapods. As such, they are important for the study of evolutionary processes involved in the water to land transition of vertebrates. The evolution of a true neck, that is, the complete separation of the pectoral girdle from the cranium, is one of the most intriguing morphological transitions known among vertebrates. Other salient changes involve new adaptations for terrestrial feeding, which involves both the cranium and its associated musculature. Historically, the cranium has been extensively investigated, but the development of the cranial muscles much less so. Here, we present a detailed study of cephalic muscle development in the Australian lungfish, Neoceratodus forsteri, which is considered to be the sister taxon to all other extant lungfishes. Neoceratodus shows several developmental patterns previously described in other taxa; the tendency of muscles to develop from anterior to posterior, from their region of origin toward insertion, and from lateral to ventral/medial (outside‐in), at least in the branchial arches. The m.protractor pectoralis appears to develop as an extension of the most posterior m.levatores arcuum branchialium, supporting the hypothesis that the m.cucullaris and its derivatives (protractor pectoralis, levatores arcuum branchialium) are branchial muscles. We present a new hypothesis regarding the homology of the ventral branchial arch muscles (subarcualis recti and obliqui, transversi ventrales) in lungfishes and amphibians. Moreover, the morphology and development of the cephalic muscles confirms that extant lungfishes are neotenic and have been strongly influenced via paedomorphosis during their evolutionary history.     

Gill‐arch musculature of the quillback carpiodes cyprinus (cypriniformes: catostomidae) with a comparison to cyprinids

Although the gill‐arch osteology of Cypriniformes has been well studied, comparable works on gill‐arch musculature are scarce. The focus of previous studies has been on Cyprinidae while other families have received little or no attention. Consequently, generalizations for Cypriniformes have been made from the musculature of cyprinid gill‐arches. This study describes the gill‐arch musculature of a catostomid, the quillback Carpiodes cyprinus, and demonstrates that there are striking differences in the overall gill‐arch musculature of catostomids in comparison to cyprinids, especially in the dorsal gill‐arch region. Of the 23 muscles found in the dorsal gill‐arch region of cyprinids, only 13 were present in C. cyprinus. Muscles that are absent include adductores 1–5, levator internus 4, levator ceratobranchialis 5 accessorius, retractor ceratobranchialis 5 externus, retractor ceratobranchialis 5 internus, and the retractor ceratobranchialis 5 transversus. In the ventral gill‐arch region, the rectus communis is absent. The derived scrolling shape of the dorsal gill‐arch skeleton associated with food processing is likely related to the change in musculature. J. Morphol., 2012. © 2012 Wiley Periodicals, Inc.     

Branchial arch muscle innervation by the glossopharyngeal (IX) and vagal (X) nerves in tetraodontiformes, with special reference to muscle homologies

Branchial arch muscle innervation by the glossopharyngeal (IX) and vagal (X) nerves in 10 tetraodontiform families and five outgroup taxa was examined, with special reference to muscle homologies. Basic innervation patterns and their variations were described for all muscle elements (except gill filament muscles). In the tetraodontids Takifugu poecilonotus and Canthigaster rivulata, diodontid Diodon holocanthus, and molid Mola mola, levator externus 4 was innervated by the 3rd vagal branchial trunk (BX3) in addition to BX2, owing to strong posterior expansion of the muscle. Based on nerve innervation, migrations of the muscle attachment sites (i.e., origins and insertions) were recognized in levator internus 2 (in Mola mola), obliquus dorsalis 3 (in Ostracion immaculatus and Canthigaster rivulata), and obliquus ventralis 2 (in Stephanolepis cirrhifer), muscle topologies not necessarily being indicative of homologies. Embryonic origin of the retractor dorsalis and parallel attainment of the swimbladder muscle within the order were also discussed.     

Functional morphology of the pharyngeal jaw apparatus in perciform fishes: An experimental analysis of the haemulidae

A new mechanical model for function of the pharyngeal jaw apparatus in generalized perciform fishes is developed from work with the family Haemulidae. The model is based on anatomical observations, patterns of muscle activity during feeding (electromyography), and the actions of directly stimulated muscles. The primary working stroke of the pharyngeal apparatus involves simultaneous upper jaw depression and retraction against a stabilized and elevating lower jaw. The working stroke is characterized by overlapping activity in most branchial muscles and is resolved into three phases. Four muscles (obliquus dorsalis 3, levator posterior, levator externus 3/4, and obliquus posterior) that act to depress the upper jaws become active in the first phase. Next, the retractor dorsalis, the only upper jaw retracting muscle, becomes active. Finally, there is activity in several muscles (transversus ventrales, pharyngocleithralis externus, pharyngohyoideus, and protractor pectoralis) that attach to the lower jaws. The combined effect of these muscles is to elevate and stabilize the lower jaws against the depressing and retracting upper jaws. The model identifies a novel mechanism of upper jaw depression, here proposed to be the primary component of the perciform pharyngeal jaw bite. The key to this mechanism is the joint between the epibranchial and toothed pharyngobranchial of arches 3 and 4. Dorsal rotation of epibranchials 3 and 4 about the insertion of the obliquus posterior depresses the lateral border of pharyngobranchials 3 and 4 (upper jaw). The obliquus dorsalis 3 muscle crosses the epibranchial-pharyngo-branchial joint in arches 3 and 4, and several additional muscles effect epibranchial rotation. Five upper jaw muscles cause upper jaw depression upon electrical stimulation: the obliquus dorsalis 3, levator posterior, levator externus 3/4, obliquus posterior, and transversus dorsalis. This result directly contradicts previous interpretations of function for the first three muscles. The presence of strong depression of the upper pharyngeal jaws explains the ability of many generalized perciform fishes to crush hard prey in their pharyngeal apparatus.     

Homologies of the branchial arch muscles in Zacco platypus (Teleostei: Cypriniformes: Cyprinidae): Evidence from innervation pattern

Homologies of the branchial arch muscles in the cyprinid Zacco platypus are assessed based on their innervation. Muscles serving the first gill arch are innervated by branches of the glossopharyngeal (IX) nerve and those serving other arches by the vagal (X) nerve. Absence of the levator posterior is confirmed. Five pairs of muscles originating from the cranium and inserted onto the specialized 5th ceratobranchial, all unique to cyprinids, are innervated by the 4th branchial trunks of X, indicating that all pairs are derivatives of the sphincter oesophagi, involving reorganization from intrinsic to extrinsic elements. Homologies of some ventral branchial muscles are also discussed and the criteria for homology improved by clarifying the innervation pattern. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Development of oral and branchial muscles in lancelet larvae of Branchiostoma japonicum

The perforated pharynx has generally been regarded as a shared characteristic of chordates. However, there still remains phylogenetic ambiguity between the cilia‐driven system in invertebrate chordates and the muscle‐driven system in vertebrates. Giant larvae of the genus Asymmetron were reported to develop an orobranchial musculature similar to that of vertebrates more than 100 years ago. This discovery might represent an evolutionary link for the chordate branchial system, but few investigations of the lancelet orobranchial musculature have been completed since. We studied staged larvae of a Japanese population of Branchiostoma japonicum to characterize the developmental property of the orobranchial musculature. The larval mouth and the unpaired primary gills develop well‐organized muscles. These muscles function only as obturators of the openings without antagonistic system. As the larval mouth enlarged posteriorly to the level of the ninth myomere, the oral musculature was fortified accordingly without segmental patterning. In contrast, the iterated branchial muscles coincided with the dorsal myomeric pattern before metamorphosis, but the pharynx was remodeled dynamically irrespective of the myomeric pattern during metamorphosis. The orobranchial musculature disappeared completely during metamorphosis, and adult muscles in the oral hood and velum, as well as on the pterygial coeloms developed independently. The lancelet orobranchial musculature is apparently a larval adaptation to prevent harmful intake. However, vestigial muscles appeared transiently with the secondary gill formation suggest a bilateral ancestral state of muscular gills, and a segmental pattern of developing branchial muscles without neural crest and placodal contributions is suggestive of a precursor of vertebrate branchiomeric pattern. J. Morphol. 275:465–477, 2014. © 2013 Wiley Periodicals, Inc.     

Morphology of the Parrotfish Pharyngeal Jaw Apparatus

SYNOPSIS. Analysis of the anatomy of the pharyngeal apparatusof parrotfish demonstrates extraordinary specialization of thegrinding jaws. The epibranchials have lost their gill-bearingfunction. The first epibranchial is the structural element ofthe pharyngeal valve that is operated by the first levator externus,first branchial adductor and part one of the transversus dorsalismuscles. Five pairs of muscles (fourth levator externus, levatorposterior lateralis and medialis, fifth branchial adductor,part two of the transversus ventralis) are positioned to adductthe lower pharyngeal. The retractor dorsalis and fourth obliquusdorsalis are positioned to retract the upper pharyngeal. Thethird levator internus and transversus dorsalis posterior protractthe upper pharyngeal. The fourth levator externus, both partsof the levator posterior and the fifth adductor are massiveand pinnate. Deep fossae for the attachment of the fourth levatorexternus and levator posterior muscles are sculpted out of theneurocranium. A ventral spike process of the prootic and expandedhemal postzygapophyses of the first three vertebrae are skeletalfeatures associated with the elaborated musculature of the pharynx.Synovial joints are present between the basicranium and upperpharyngeals, between the upper pharyngeals and fourth epibranchialsand between the lower pharyngeal and cleithrum. The upper pharyngealsact as a single unit bound by cruciate ligaments. The fourthepibranchial is a key element in the pharyngeal apparatus andserves to direct forces generated by the transversus ventralis,fifth adductor, levator posterior lateralis, transversus dorsalisposterior and fourth obliquus dorsalis.     

The hyal and ventral branchial muscles in caecilian and salamander larvae: Homologies and evolution

Amphibians (Lissamphibia) are characterized by a bi‐phasic life‐cycle that comprises an aquatic larval stage and metamorphosis to the adult. The ancestral aquatic feeding behavior of amphibian larvae is suction feeding. The negative pressure that is needed for ingestion of prey is created by depression of the hyobranchial apparatus as a result of hyobranchial muscle action. Understanding the homologies of hyobranchial muscles in amphibian larvae is a crucial step in understanding the evolution of this important character complex. However, the literature mostly focuses on the adult musculature and terms used for hyal and ventral branchial muscles in different amphibians often do not reflect homologies across lissamphibian orders. Here we describe the hyal and ventral branchial musculature in larvae of caecilians (Gymnophiona) and salamanders (Caudata), including juveniles of two permanently aquatic salamander species. Based on previous alternative terminology schemes, we propose a terminology for the hyal and ventral branchial muscles that reflects the homologies of muscles and that is suited for studies on hyobranchial muscle evolution in amphibians. We present a discussion of the hyal and ventral branchial muscles in larvae of the most recent common ancestor of amphibians (i.e. the ground plan of Lissamphibia). Based on our terminology, the hyal and ventral branchial musculature of caecilians and salamanders comprises the following muscles: m. depressor mandibulae, m. depressor mandibulae posterior, m. hyomandibularis, m. branchiohyoideus externus, m. interhyoideus, m. interhyoideus posterior, m. subarcualis rectus I, m. subarcualis obliquus II, m. subarcualis obliquus III, m. subarcualis rectus II‐IV, and m. transversus ventralis IV. Except for the m. branchiohyoideus externus, all muscles considered herein can be assigned to the ground plan of the Lissamphibia with certainty. The m. branchiohyoideus externus is either apomorphic for the Batrachia (frogs + salamanders) or salamander larvae depending on whether or not a homologous muscle is present in frog tadpoles. J. Morphol., 2011. © 2011 Wiley‐Liss, Inc.     

Mandibular arch musculature of anuran tadpoles, with comments on homologies of amphibian jaw muscles

This study analyzes the structure of the mandibular arch musculature in larval, metamorphic, and postmetamorphic anurans of 26 species and makes comparisons with larvae of three caudate and one gymnophione species. Major transformations in early evolution of anuran larvae comprise, for example, the powering of the larval upper jaw cartilages by relocating insertion sites of mandibular arch levators; splitting of some larval muscles into two muscles or muscle heads (m. intermandibularis, m. lev. mand. externus, m. lev. mand. longus); evolution of a muscle invading the lower lip of the oral disk (m. mandibulolabialis), and shift of origin of the internus and longus muscles from dorsal on the cranium to sites on the ventral otic capsule and palatoquadrate, respectively. In all these characters, Ascaphus truei shares the plesiomorphic conditions with caudates. The larva of Xenopus laevis is remarkable because the insertion pattern of three larval mandibular muscles anticipates the postmetamorphic condition of frogs in general and also resembles the caudate condition. Discoglossids, bombinatorids, pelobatids, and neobatrachians are largely similar in their muscle arrangements. The filter-feeding microhylids, however, have most clearly modified the general neobatrachian pattern. Past conflicts in the interpretation and naming of muscles can be attributed to the implicit or explicit homology assumptions used. In particular, the muscles' relations to the branches of the trigeminal nerve have been the dominant criteria for inferring homology and has led to inconsistencies. This concept is questioned herein. It is observed that the relative position of the ramus mandibularis (V(3)) is more variable interspecifically in anuran larvae than previously thought. The relations of the nerve branches and muscles in larvae are maintained during metamorphosis. Considering the muscle pattern to be more conserved in interspecific comparisons than the position of the nerve branches results in a new interpretation of muscle homologies and a hypothesis of jaw muscle evolution in amphibians that is more parsimonious than earlier views. A new, simplified terminology for the jaw musculature is proposed that is applicable for larvae and adults. It maximizes information content and reflects the hypothesized homologies of amphibian jaw muscles.     

Cranial muscle development in frogs with different developmental modes: Direct development versus biphasic development

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana (Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved. J. Morphol. 275:398–413, 2014. © 2013 Wiley Periodicals, Inc.     

Ontogenetic differences in the feeding biomechanics of oviparous and viviparous caecilians (Lissamphibia: Gymnophiona)

Caecilians have a unique dual jaw-closing system in that jaw closure is driven by the ancestral jaw-closing muscles (mm. levatores mandibulae) plus a secondarily recruited hyobranchial muscle (m. interhyoideus posterior). There is a variety of feeding habits (suction feeding, skin feeding, intrauterine scraping, and biting) during ontogeny that relate to reproductive modes in different caecilian species. This study examines the cranial biomechanics of caecilians in the suction-feeding larva of Ichthyophis cf. kohtaoensis, in the embryo and juvenile of the skin-feeding Boulengerula taitana, and in a newborn of the intrauterine feeder Typhlonectes natans. A lever arm model was applied to calculate effective mechanical advantages of jaw-closing muscles over gape angles and to predict total bite force in developing caecilians. In I. cf. kohtaoensis, Notable differences were found in the larval jaw-closing system compared to that of the adult. The suction-feeding larva of I. cf. kohtaoensis has comparatively large mm. levatores mandibulae that insert with an acute muscle fiber angle to the lower jaw and a m. interhyoideus posterior that has its optimal leverage at small gape angles. Conversely, the skin-feeding juvenile of B. taitana and the neonate T. natans are very similar in the feeding parameters considered herein compared to adult caecilians. Some ontogenetic variation in the feeding system of B. taitana before the onset of feeding was present. This study contributes to our understanding of the functional demands that feeding habits put on the development of cranial structures.     

Gill microcirculation of the air-breathing climbing perch, Anabas testudineus (Bloch):relationships with the accessory respiratory organs and systemic circulation

The general macrocirculation and branchial microcirculation of the air-breathing climbing perch, Anabas testudineus, was examined by light and scanning electron microscopy of vascular corrosion replicas. The ventral aorta arises from the heart as a short vessel that immediately bifurcates into a dorsal and a ventral branch. The ventral branch distributes blood to gill arches 1 and 2, the dorsal branch to arches 3 and 4. The vascular organization of arches 1 and 2 is similar to that described for aquatic breathing teleosts. The respiratory lamellae are well developed but lack a continuous inner marginal channel. The filaments contain an extensive nutritive and interlamellar network; the latter traverses the filament between, but in register with, the inner lamellar margins. Numerous small, tortuous vessels arise from the efferent filamental and branchial arteries and anastomose with each other to form the nutrient supply for the filament, adductor muscles, and arch supportive tissues. The efferent branchial arteries of arches 1 and 2 supply the accessory air-breathing organs. Arches 3 and 4 are modified to serve primarily as large-bore shunts between the dorsal branch of the ventral aorta and the dorsal aorta. In many filaments from arches 3 and 4, the respiratory lamellae are condensed and have only 1-3 large channels. In some instances in arch 4, shunt vessels arise from the afferent branchial artery and connect directly with the efferent filamental artery. The filamental nutrient and interlamellar systems are poorly developed or absent. The respiratory and systemic pathways in Anabas are arranged in parallel. Blood flows from the ventral branch of the ventral aorta, through gill arches 1 and 2, into the accessory respiratory organs, and then returns to the heart. Blood, after entering the dorsal branch of the ventral aorta, passes through gill arches 3 and 4 and proceeds to the systemic circulation. This arrangement optimizes oxygen delivery to the tissues and minimizes intravascular pressure in the branchial and air-breathing organs. The efficiency of this system is limited by the mixing of respiratory and systemic venous blood at the heart.     

Homologies of the longissimus, iliocostalis, and hypaxial muscles in the anterior presacral region of extant diapsida

Homologies of muscles of the m. longissimus and m. iliocostalis groups in the dorsal and cervical regions, as well as those of the subvertebral muscles and mm. intercostales externi that continue from the dorsal into the cervical regions, in extant Diapsida are proposed based on detailed dissections and published accounts of lepidosaurs, crocodylians, and birds. The morphology of tendons and innervation patterns suggest that the avian "m. iliocostalis" in the dorsal region include the homologs of both m. longissimus and m. iliocostalis in non-avian diapsids. The conserved nature of the morphology of tendons in palaeognath birds also revealed that the avian mm. intertransversarii in the cervical region consist of muscles of the both m. longissimus and m. iliocostalis groups despite having been treated as a single series of muscles, and thus are not homologous with muscles of the same name in Lepidosauria or Crocodylia. The avian mm. inclusi that lie medial to mm. intertransversarii are homologous with mm. intercostales externi in Lepidosauria and mm. intercostales externi and m. scalenus combined in Crocodylia. Innervation patterns suggest that a muscle ("m. iliocostalis capitis") connecting the atlas rib and occiput in Crocodylia includes contributions from the subvertebral layer and m. cucullaris complex, and possibly m. iliocostalis as well. The present findings may serve as a basis for revising the currently used avian nomenclature so that it will reflect homologies of muscles with their non-avian counterparts.     

Cephalic muscles of Cyclostomes (hagfishes and lampreys) and Chondrichthyes (sharks,rays and holocephalans): comparative anatomy and early evolution of the vertebrate head muscles

Living vertebrate diversity comprises hagfishes and lampreys (Cyclostomata), elasmobranchs and holocephalans (Chondrichthyes), and bony fish which include tetrapods (Osteichthyes). Based on dissections and an extensive comparative analysis, we provide an updated overview of the anatomy, homologies and evolution of cyclostome and chondrichthyan cephalic muscles, with osteichthyans as primary comparative taxa. The analysis also infers plesiomorphic conditions for vertebrates and gnathostomes. We follow a uniform myological terminology for the Gnathostomata to demonstrate that the last common ancestor of extant vertebrates probably had a single intermandibularis and other mandibular muscles (labial muscles), some constrictores hyoidei and branchiales, and epibranchial and hypobranchial muscle sheets. The division of the cucullaris into levatores arcuum branchialium and protractor pectoralis is an osteichthyan synapomorphy and reflects an evolutionary trend towards a greater separation between the head and pectoral girdle that culminated in the formation of the tetrapod neck. Hence, this paper addresses a long‐standing, central issue regarding vertebrate comparative anatomy. It thus provides a valuable basis for future evolutionary, developmental and functional studies of vertebrates and/or of specific vertebrate subgroups/model organisms. © 2014 The Linnean Society of London     

Description of the muscular system of the fantail <Emphasis Type="Italic">Carassius auratus gibelio</Emphasis> (Cyprinidae)

The muscular system of a variety of the goldfish Carassius auratus gibelio fantail is investigated and described in detail for the first time. The structure of the muscular system principally corresponds to that of other Teleostei. At the same time, in contrast to other fish, in the fantail three posterior lower-keel muscles are found for the first time, which depends on bifurcation of the caudal and anal fins. Two of them correspond to the usual posterior lower-keel muscles and proceed along the edges of the lower part of the caudal peduncle. The third muscle termed m. infracarinalis posterior medianus (median posterior lower-keel muscle) proceeds in the middle between them. The second distinction of the fantail from other fish, including the carp Cyprinus carpio of the same family Cyprinidae, is the absence of some muscles of the upper parts of the gill arches, such as dorsal rectus muscles (m.m. recti dorsales), adductor muscles of gill arches (m.m. adductores arcuum branchialium), and oblique dorsal muscles (m.m. obliqui dorsales).     

Major muscle systems in the larval caenogastropod,Ilyanassa obsoleta,display different patterns of development

This study describes the anatomical and developmental aspects of muscular development from the early embryo to competent larval stage in the gastropod Ilyanassa obsoleta. Staining of F‐actin revealed differential spatial and temporal patterns of several muscles. In particular, two major muscles, the larval retractor and pedal retractor muscles originate independently and display distinct developmental patterns similar to observations in other gastropod species. Additionally, together with the larval retractor muscle, the accessory larval muscle developed in the embryo at the trochophore stage. Therefore, both these muscles develop prior to ontogenetic torsion. The pedal retractor muscle marked the most abundant growth in the mid veliger stage. Also during the middle stage, the metapodial retractor muscle and opercular retractor muscle grew concurrently with development of the foot. We show evidence that juvenile muscles, such as the buccal mass muscle and siphon muscle develop initially during the late veliger stage. Collectively, these findings substantiate that larval myogenesis involves a complex sequence of events that appear evolutionary conserved within the gastropods, and set the stage for future studies using this model species to address issues concerning the evolution and eventual fates of larval musculature in molluscs. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Myogenesis in two polyclad platyhelminths with indirect development,Pseudoceros canadensis and Stylostomum sanjuania

SUMMARY Myogenesis of two representatives of Platyhelminthes, Stylostomum sanjuania and Pseudoceros canadensis, was followed from egg deposition until well‐differentiated free‐swimming larval stages, using F‐actin staining and confocal laserscanning microscopy. Zonulae adhaerentes are the only structures to stain before 50% of development between egg deposition and hatching in S. sanjuania, and before 67% of development in P. canadenis. Subsequently, irregular fibers appear in the embryo, followed by a helicoid muscle close to the apical pole. Three longitudinal muscle pairs form, of which the dorsal pair remains more pronounced than the others. Gradually, new muscles form by branching or from double‐stranded muscle zones adjacent to existing muscles. This results in an elaborate muscular bodywall that consists of a single helicoid muscle as well as multiple circular and longitudinal muscles. Diverse retractor muscles insert at the sphincter muscles around the stomodeum. The overall arrangement and formation mode of the larval musculature appears very similar in both species, although only P. canadensis has a primary circular muscle posterior to the helicoid muscle. Muscle formation in the apical region of the embryo precedes that at the abapical pole and the primary longitudinal muscles form slightly later than the primary circular muscles. Myogenesis and larval myoanatomy appears highly conserved among polyclad flatworms, but differs significantly from that of other trochozoan clades. Our data suggest that the larval muscular ground pattern of polyclad larvae comprises a bodywall consisting of a helicoid muscle, circular and longitudinal muscles, several retractor muscles, and sphincter muscles around the stomodeum.     

Comparative morphology and ultrastructure of the mouthparts in unfed larvae of <Emphasis Type="Italic">Platytrombidium fasciatum</Emphasis> and <Emphasis Type="Italic">Camerotrombidium pexatum</Emphasis> (Acariformes: Microtrombidiidae)

The mouthparts of unfed larvae of Platytrombidium fasciatum (C. L. Koch, 1836) and Camerotrombidium pexatum (C. L. Koch, 1837) (Acariformes: Microtrombidiidae) were studied using both light optical (whole-mounted specimens, toluidine blue stained semi-thin sections) and electron microscope (TEM, SEM) methods. The mouth apparatus incorporated within the gnathosoma occupies an axial position and is covered from above by the arched dorsal shield, or scutum. The chelicerae are comparatively long and separated, and the lateral lips form a permanent apomorphic sucker provided with an internal cuticular sclerite. The pharynx is extremely wide and totally fused with the bottom of the infracapitulum. The pharyngeal dilators originate on the posterior portions of the cervix (epistome) and on the capitular apodemes and run nearly parallel to the cervix to the dorsal pharyngeal wall. Comparatively short sigmoid pieces serve as origin of the muscles—cheliceral levators inserting on the posterior wall of the basal cheliceral segments. There are two sets of the extrinsic gnathosomal muscles originating on the posterior portion of the scutum: retractors of chelicerae inserting on the posterior portions of the basal cheliceral segments, and retractors of the gnathosoma inserting on the very posterior parts of the capitular apodemes. The labrum and the cervix delimit the pharynx and the subcheliceral space. The labrum and the cervix for the most part are weakly sclerotized cuticular plates and do not have own muscles. The larval mouth apparatus, in comparison with that of adult microtrombidiid mites, is simply organized and more specialized for ingestion of large masses of liquid food.