Locomotory aspects of squid mantle structure

Morphological aspects of squid ( Loligo, Lolliguncula ) mantle relevant to locomotory function were studied. Methods used included polarized light microscopy of frozen sections of untreated tissue taken from animals immediately after death and electron microscopy.
The mantle consists of circular and radial muscles arranged in alternating rings along the whole length of the mantle. The muscle is obliquely striated. Connective tissue fibres are found in the body of the muscle and in the outer and inner tunics. The outer tunic consists of layers of large collagenous fibres. The fibres run in superimposed right- and left-handed helical courses that lie at an angle of 27° to the long axis of the animal. The tunics and the intramuscular connective fibres are thought to resist length changes in the mantle while permitting the changes in girth required for the jet power stroke. Both the intramuscular and the tunic fibre systems may provide elastic energy for the return phase of the jet cycle. Tunic fibres appear to be a geodesic tensile reinforcing system ensuring smooth shape changes in the mantle.     

Limitations on locomotor performance in squid

An empirical equation relating O2 consumption (power input) to pressure production during jet-propelled swimming in the squid (Illex illecebrosus) is compared with hydrodynamic estimates of the pressure-flow power output also calculated from pressure data. Resulting estimates of efficiency and stress indicate that the circularly arranged obliquely striated muscles in squid mantle produce maximum tensions about half those of vertebrate cross-striated muscle, that "anaerobic" fibers contribute to aerobic swimming, and that peak pressure production requires an instantaneous power output higher than is thought possible for muscle. Radial muscles probably contribute additional energy via elastic storage in circular collagen fibers. Although higher rates of aerobic power consumption are only found in terrestrial animals at much higher temperatures, the constraint on squid performance is circulation, not ventilation. Anaerobic power consumption is also among the highest ever measured, but the division of labor between "aerobic" and "anaerobic" fibers suggests a system designed to optimize the limited capacity of the circulation.     

Gradients of strain and strain rate in the hollow muscular organs of soft-bodied animals

The cylindrical shape of soft-bodied invertebrates is well suited to functions in skeletal support and locomotion, but may result in a previously unrecognized cost—large non-uniformities in muscle strain and strain rate among the circular muscle fibres of the body wall. We investigated such gradients of strain and strain rate in the mantle of eight long-finned squid Doryteuthis pealeii and two oval squid Sepioteuthis lessoniana. Transmural gradients of circumferential strain were present during all jets (n = 312); i.e. for a given change in the circumference of the outer surface of the mantle, the inner surface experienced a greater proportional change. The magnitude of the difference increased with the amplitude of the mantle movement, with circular muscle fibres at the inner surface of the mantle experiencing a total range of strains up to 1.45 times greater than fibres at the outer surface during vigorous jets. Differences in strain rate between the circular fibres near the inner versus the outer surface of the mantle were also present in all jets, with the greatest differences occurring during vigorous jetting. The transmural gradients of circumferential strain and strain rate we describe probably apply not only to squids and other coleoid cephalopods, but also to diverse soft-bodied invertebrates with hollow cylindrical or conical bodies and muscular organs.     

The structure and adhesive mechanism of octopus suckers

Octopus suckers consist of a tightly packed three-dimensionalarray of muscle with three major muscle fiber orientations:1) radial muscles that traverse the wall; 2) circular musclesarranged circumferentially around the sucker; and 3) meridionalmuscles oriented perpendicular to the circular and radial muscles.The sucker also includes inner and outer fibrous connectivetissue layers and an array of crossed connective tissue fibersembedded in the musculature. Adhesion results from reducingthe pressure inside the sucker cavity. This can be achievedby the three-dimensional array of muscle functioning as a muscular-hydrostat.Contraction of the radial muscles thins the wall, thereby increasingthe enclosed volume of the sucker. If the sucker is sealed toa surface the cohesiveness of water resists this expansion.Thus, the pressure of the enclosed water decreases instead.The meridional and circular muscles antagonize the radial muscles.The crossed connective tissue fibers may store elastic energy,providing an economical mechanism for maintaining attachmentfor extended periods. Measurements using miniature flush-mountedpressure transducers show that suckers can generate hydrostaticpressures below 0 kPa on wettable surfaces but cannot do soon non-wettable surfaces. Thus, cavitation, the failure of waterin tension, may limit the attachment force of suckers. As depthincreases, however, cavitation will cease to be limiting becauseambient pressure increases with depth while the cavitation thresholdis unchanged. Structural differences between suckers will thendetermine the attachment force.     

Musculature of the penial complex: A new criterion in unravelling the phylogeny of Hygrophila (Gastropoda: Pulmonata)

The taxonomy of freshwater pulmonates (Hygrophila) has been in a fluid state warranting the search for new morphological criteria that may show congruence with molecular phylogenetic data. We examined the muscle arrangement in the penial complex (penis and penis sheath) of most major groups of freshwater pulmonates to explore to which extent the copulatory musculature can serve as a source of phylogenetic information for Hygrophila. The penises of Acroloxus lacustris (Acroloxidae), Radix auricularia (Lymnaeidae), and Physella acuta (Physidae) posses inner and outer layers of circular muscles and an intermediate layer of longitudinal muscles. The inner and outer muscle layers in the penis of Biomphalaria glabrata consist of circular muscles, but this species has two intermediate longitudinal layers separated by a lacunar space, which is crossed by radial and transverse fibers. The muscular wall of the penis of Planorbella duryi is composed of transverse and longitudinal fibers, with circular muscles as the outer layer. In Planorbidae, the penial musculature consists of inner and outer layers of longitudinal muscles and an intermediate layer of radial muscles. The penis sheath shows more variation in muscle patterns: its muscular wall has two layers in A. lacustris, P. acuta, and P. duryi, three layers in R. auricularia and Planorbinae and four layers in B. glabrata. To trace the evolution of the penial musculature, we mapped the muscle characters on a molecular phylogeny constructed from the concatenated 18S and mtCOI data set. The most convincing synapomorphies were found for Planorbinae (inner and outer penis layers of longitudinal muscles, three-layered wall of the penis sheath). A larger clade coinciding with Planorbidae is defined by the presence of radial muscles and two longitudinal layers in the penis. The comparative analysis of the penial musculature appears to be a promising tool in unraveling the phylogeny of Hygrophila.     

Effects of prey type on motor pattern variance in tetraodontiform fishes

It is unclear whether the high variance of electromyographic parameters measured in feeding teleost fishes reflects functionally significant motor variation that is under control of the fish, or functionally insignificant variation characteristic of EMG data. We addressed this issue by examining the effect of three prey, differing in physical characteristics, on the feeding motor pattern in three fishes of the Order Tetraodontiformes: the filefish, Monacanthus hispidus; the triggerfish, Balistes capriscus; and the puffer, Sphoeroides nephelus. EMG recordings were made from subdivisions of the mouth closing adductor mandibulae muscle and the mouth opening levator operculi muscle in four fish from each species feeding on live fiddler crabs, live shrimp, and pieces of cut squid mantle. Analysis of variance was used to test for effects of prey type on the standard deviation of muscle burst duration, burst onset time, and average burst amplitude in the adductor muscles. The filefish exhibited a doubling of standard deviation of burst duration in all muscles when feeding on fiddler crabs; triggerfish showed increased standard deviations in onset times and duration of two muscles when feeding on squid mantle; and the puffer showed no effects of prey on motor variability. The observation that prey type can elicit more than a doubling in the standard deviation of some EMG traits indicates that a large portion of the within-prey type variance is under direct control of the individual fish, suggesting an even greater level of fine motor control in teleost feeding mechanisms than previously recognized.     

Ontogeny of squid mantle function: changes in the mechanics of escape-jet locomotion in the oval squid,Sepioteuthis lessoniana lesson, 1830

In Sepioteuthis lessoniana, the oval squid, ontogenetic changes in the kinematics of the mantle during escape-jet locomotion imply a decline in the relative mass flux of the escape jet and may affect the peak weight-specific thrust of the escape jet. To examine the relationship between ontogenetic changes in the kinematics of the mantle and the thrust generated during the escape jet, we simultaneously measured the peak thrust and the kinematics of the mantle of squid tethered to a force transducer. We tested an ontogenetic series of S. lessoniana that ranged in size from 5 to 40 mm dorsal mantle length (DML). In newly hatched squids, thrust peaked 40 ms after the start of the escape jet and reached a maximum of between 0.10 mN and 0.80 mN. In the largest animals, thrust peaked 70 ms after the start of the escape jet and reached a maximum of between 18 mN and 110 mN. Peak thrust was normalized by the wet weight of the squid and also by the cross-sectional area of the circumferential muscle that provides power for the escape jet. The weight-specific peak thrust of the escape jet averaged 0.36 in newly hatched squid and increased significantly to an average of 1.5 in the largest squids measured (P < 0.01). The thrust per unit area of circumferential muscle averaged 0.25 mN/mm(2) in hatchlings and increased significantly to an average of 1.4 mN/mm(2) in the largest animals tested (P < 0.01). The impulse of the escape jet was also lowest in newly hatched individuals (1.3 mN. s) and increased significantly to 1000 mN. s in the largest squids measured (P < 0.01). These ontogenetic changes in the mechanics of the escape jet suggest (1) that propulsion efficiency of the exhalant phase of the jet is highest in hatchlings, and (2) that the mechanics of the circumferential muscles of the mantle change during growth.     

Role of aerobic and anaerobic circular mantle muscle fibers in swimming squid: electromyography

Circular mantle muscle of squids and cuttlefishes consists of distinct zones of aerobic and anaerobic muscle fibers that are thought to have functional roles analogous to red and white muscle in fishes. To test predictions of the functional role of the circular muscle zones during swimming, electromyograms (EMGs) in conjunction with video footage were recorded from brief squid Lolliguncula brevis (5.0-6.8 cm dorsal mantle length, 10.9-18.3 g) swimming in a flume at speeds of 3-27 cm s(-1). In one set of experiments, in which EMGs were recorded from electrodes intersecting both the central anaerobic and peripheral aerobic circular mantle muscles, electrical activity was detected during each mantle contraction at all swimming speeds, and the amplitude and frequency of responses increased with speed. In another set of experiments, in which EMGs were recorded from electrodes placed in the central anaerobic circular muscle fibers alone, electrical activity was not detected during mantle contraction until speeds of about 15 cm s(-1), when EMG activity was sporadic. At speeds greater than 15 cm s(-1), the frequency of central circular muscle activity subsequently increased with swimming speed until maximum speeds of 21-27 cm s(-1), when muscular activity coincided with the majority of mantle contractions. These results indicate that peripheral aerobic circular muscle is used for low, intermediate, and probably high speeds, whereas central anaerobic circular muscle is recruited at intermediate speeds and used progressively more with speed for powerful, unsteady jetting. This is significant because it suggests that there is specialization and efficient use of locomotive muscle in squids.     

Myosin-linked calcium regulation in squid mantle muscle. Light-chain components of squid myosin.

As reported by Kendrick-Jones et al. (1976), myosin from squid mantle muscle contains two types of light-chain components, different in size but similar in net charge. We were able to separate the two types of light chains by a five-step procedure, yielding LC-1 (17,000 daltons) and LC-2 (15,000 daltons). It was found that squid mantle LC-1 and LC-2 function exactly like SH-light chains and EDTA-light chains of scallop adductor myosin, respectively. In functional tests, we used "desensitized" myosin of scallop adductor muscle, simply because "EDTA washing" removed neither LC-1 nor LC-2 from squid mantle myosin. The removal and recombination of light chains were examined by gel electrophoresis, and Ca or Sr sensitivity was determined by measuring the Mg-ATPase activity of skeletal acto-scallop or squid myosin. It was found that EDTA washing readily released the EDTA-light chains of scallop myosin completely, and that the EDTA-washed scallop myosin was capable of regaining its full content of EDTA-LC as well as its full sensitivity to calcium. We also found that as regards combining with, and conferring calcium sensitivity on the EDTA-washed myosin of scallop adductor, squid mantle LC-2 could effectively replace scallop adductor EDTA-LC. In addition, calcium or strontium ions were found to induce changes in the UV absorption spectrum of scallop adductor EDTA-LC, although the apparent binding constants estimated from the difference spectrum were too low to account for the Ca or Sr sensitivity of scallop actomyosin-ATPase. The divalent cations also induced changes in the UV absorption spectrum of squid LC-2, and the apparent binding constants estimated from the difference spectrum were sufficiently high (1.5 X 10(5) M-1 for Ca binding, and 1.6 X 10(3) M-1 for Sr binding) to account for the Ca and Sr sensitivities of squid mantle myosin B-ATPase. The findings with scallop adductor myosin are in conflict with those reported by Kendrick-Jones et al., and must be accounted for in formulating the molecular mechanism of myosin-linked calcium regulation in molluscan muscles.     

Body wall muscles in oligochaetes

Longitudinal body wall muscles in tubificids, enchytraeids, lumbriculids, haplotaxids and lumbricids are always histologically and ultrastructurally distinguishable from one another. Some structural peculiarities, as those typical of lumbriculids, may be used as taxonomic tools. The longitudinal body wall muscles in all oligochaetes belong to slow and fast contracting fibre types. Two possible phylogenetic patterns for the formation of slow fibres are discussed.     

Thermodynamic characterization of muscle tropomyosins from marine invertebrates

Structural properties of invertebrate muscle tropomyosin (TM) have not been characterized in detail to date. TMs were thus purified from the mantle muscle of Japanese common squid Todarodes pacificus, the foot muscle of tokobushi abalone Haliotis diversicolor and the tail muscle of kuruma prawn Marsupenaeus japonicus, and investigated for their thermodynamic properties by circular dichroism (CD) spectrometry and differential scanning calorimetry (DSC). From the CD spectrometry data, the apparent melting temperature and the apparent free energy of unfolding at 20°C were calculated to be 43.5°C and 14.5 kJ/mol for the squid TM, 43.0°C and 23.9 kJ/mol for the abalone TM, and 47.3°C and 50.2 kJ/mol for the prawn TM, respectively. From the DSC data, the total free energy of unfolding at 20°C was calculated to be 129 kJ/mol, 253 kJ/mol, and 271 kJ/mol for the squid, abalone, and prawn TMs, respectively. These results suggest that the thermal stability was in the order of prawn TM>abalone TM>squid TM.     

The neuroactive substances and associated muscle system in Rhipidocotyle campanula (Digenea,Bucephalidae) from the intestine of the pike Esox lucius

We report about the muscular system and the serotonergic and FMRFamidergic components of the nervous system of the Bucephalidae trematode, Rhipidocotyle campanula, an intestinal parasite of the pike. We use immunocytochemical methods and confocal scanning laser microscopy (CLSM). The musculature is identified by histochemical staining with fluorescently labeled phalloidin. The body wall musculature of R. campanula contains three layers of muscle fibres – the outer thin circular, intermediate longitudinal and inner diagonal muscle fibres running in two opposite directions. The digestive system of R. campanula possess of a well-developed musculature: radial, longitudinal and circular muscle elements are detected in the pharynx, circular and longitudinal muscle filaments seen in the oesophagus, and longitudinal and the circular muscle fibres were found in the intestinal wall. Specific staining indicating the presence of actin muscle filaments occurs in the cirrus sac localized in the posterior body region. The frontal region of anterior attachment organ, the rhynchus, in R. campanula is represented by radial muscle fibres. The posterior part of the rhynchus comprise of radial muscles forming the organ's wall, and several strong longitudinal muscle bundles. Serotonergic and FMRFamidergic structures are detected in the central and peripheral compartments of the nervous system of R. campanula, that is, in the paired brain ganglia, the brain commissure, the longitudinal nerve cords, and connective nerve commissures. The innervations of the rhynchus, pharynx, oesophagus and distal regions of the reproductive system by the serotonergic and FMRFamidergic nervous elements are revealed. We compare our findings obtained on R. campanula with related data for other trematodes.     

Innervation of holothurian body wall muscle: inhibitory effects and localization of 5-HT

We investigated innervation to body wall muscles as well as distribution of 5-HT (serotonin) and its effects on longitudinal muscles of body wall (LMBW) of the sea cucumber Apostichopus japonicus. With serial sections we found neural branches and fibers extending from hyponeural part of radial nerve towards LMBW and circular muscles of body wall. With the aqueous aldehyde (Faglu) method yellow fluorescence indicating indolamines was observed in LMBW and in the mesentery connecting LMBW to the body wall. With indirect immunohistochemistry 5-HT-like immunoreactivity was observed in LMBW and in mesentery. These results strongly suggested that both LMBW and mesentery contained 5-HT. The effects of monoamine neurotransmitters were studied in LMBW. Putative neurotransmitters tested were 5-HT, adrenaline, noradrenaline, dopamine, and DOPA at the concentration of 10(-6) M. The application of 5-HT caused no contraction or relaxation, but it inhibited the contraction induced by 10(-6)-10(-5) M acetylcholine (ACh). Catecholamines were ineffective by themselves and had no effects on the contraction induced by ACh. The present histological, histochemical, and pharmacological studies strongly suggested that holothurian LMBW was innervated by inhibitory serotonergic neurons of the hyponeural nervous system.     

Two calcium regulation systems in squid (Ommastrephes sloani pacificus) muscle. Preparation of calcium-sensitive myosin and troponin-tropomyosin.

The Ca-regulatory system in squid mantle muscle was studied. The findings were as follows. (a) Squid mantle myosin B (squid myosin B) was Ca-sensitive, and its Ca-sensitivity was unaffected by addition of a large amount of rabbit skeletal myosin (skeletal myosin) or rabbit skeletal F-actin (skeletal F-actin). (b) Squid myosin was prepared from the mantle muscle. It showed a heavy chain component and two light chain components in the SDS-gel electrophoretic pattern: the molecular weights of the latter two were 17,000 and 15,000. Actomyosin reconstituted from squid myosin and skeletal (or squid) actin showed Ca-sensitivity in superprecipitation and Mg-ATPase assays. EDTA- treatment had no effect on the Ca-sensitivity of squid myosin. (c) Squid mantle actin (squid actin) was prepared by the method of Spudich and Watt. Hybrid actomyosin reconstituted by using the pure squid actin preparation with skeletal myosin showed no Ca-sensitivity in Mg-ATPase assay, whereas that reconstituted using crude squid actin showed marked Ca-sensitivity. The crude squid actin contained four protein components which were capable of associating with F-actin in 0.1 M KCl, 1 mM MgCl2 and 20 mM Tris-maleate (pH7.5). (d) Native tropomyosin was prepared from squid mantle muscle, and it conferred Ca-sensitivity on skeletal actomyosin as well as on a hybrid actomyosin reconstituted from squid actin and skeletal myosin. (e) Squid native tropomyosin was separated into troponin and tropomyosin fractions by placing it in 0.4 M LiCl at pH 4.7. The troponin fraction was further purified by DEAE-cellulose chromatography. Squid troponin thus obtained was different in mobility from rabbit skeletal or carp dorsal troponin; three bands of squid troponin corresponded to molecular weights of 52,000, 28,000, and 24,000 daltons. It could confer Ca-sensitivity in the presence of tropomyosin on skeletal actomyosin as well as on a hybrid reconstituted from squid actin and skeletal myosin. (f) Squid myosin B, and two hybrid actomyosins were compared as regards Ca and Sr requirements for their Mg-ATPase activities. The myosin-linked regulatory system rather than the thin-filament-linked regulatory system was predominant in squid myosin B. Squid myosin B required higher Ca2+ and Sr2+ concentrations for Mg-ATPase activity; half-maximal activation of Mg-ATPase was obtained at 0.8 micron Ca2+ and 28 micron Sr2+ with skeletal myosin B, and at 2.5 micron Ca2+ and 140 micron Sr2+ with squid myosin B.     

Ontogenetic changes in mantle kinematics during escape-jet locomotion in the oval squid, Sepioteuthis lessoniana Lesson, 1830.

We investigated the kinematics of mantle movement during escape jet behavior in an ontogenetic series of Sepioteuthis lessoniana, the oval squid. Changes in mantle diameter during the jet were measured from digitized S-VHS video fields of tethered animals that ranged in age from hatchlings to 9 weeks. The amplitude of both mantle contraction and mantle hyperinflation (expressed as percent change from the resting mantle diameter) during an escape jet was significantly greater in hatchlings than in older, larger squid (P < 0.05). The maximum amplitude of mantle contraction during the escape jet decreased from an average of -40% in hatchlings to -30% in the largest animals studied. The maximum amplitude of mantle hyperinflation decreased from an average of 18% in hatchlings to 9% in the largest squid examined. In addition, the maximum rate of mantle contraction decreased significantly during ontogeny (P < 0.05), from a maximum of 8.6 mantle circumference lengths per second (L/s) in hatchlings to 3.8 L/s in the largest animals studied. The ontogenetic changes in the mantle kinematics of the escape jet occurred concomitantly with changes in the organization of collagenous connective tissue fiber networks in the mantle. The alteration in mantle kinematics during growth may result in proportionately greater mass flux during the escape jet in newly hatched squid than in larger animals.     

On the absence of circular muscle elements in the body wall of Dysponetus pygmaeus (Chrysopetalidae, 'Polychaeta', Annelida)

The annelid body wall generally comprises an outer layer of circular muscle fibres and an inner layer of longitudinal muscle fibres as well as parapodial and chaetal muscles. An investigation of Dysponetuspygmaeus (Chrysopetalidae) with confocal laser scanning microscopy showed that circular muscles are entirely absent. Further studies indicate that this feature is characteristic for all Chrysopetalidae. A scrutiny of the literature showed a similar situation in many other polychaetes. This lack of circular muscle fibres may either be due to convergence or represent a plesiomorphic character. Since circular muscles are very likely important for burrowing forms but not necessary for animals which proceed by movements of their parapodial appendages or cilia, this problem is also related to the question of whether the ancestral polychaete was epi‐ or endobenthic.     

Vigilance and antipredator responses of Caribbean reef squid

Antipredator responses, especially those of open-ocean squid, have been seldom studied in the natural environment. Sepioteuthis sepioidea, observed by snorkellers near the shore in early morning/late afternoon, produced an average of eight moves of over 1?m per hour, apparently mostly antipredator behaviours. Close approaches by herbivorous parrotfish elicited no response in 74% of encounters; otherwise, squid produced agonistic zebra stripes or startle-mantle-dots skin patterns. Predatory bar jack fish caused flight but not zebra displays, and squid usually paled and fled quickly (66%) from snapper. The speed of approach was the best predictor for flight and display responses to snapper, but for bar jack and parrotfish, the relative fish size and distance were the predictors for escalated responses. Paired dorsolateral mantle dots were produced when squid approached the sea bottom or hunted outside the group and in reaction to fish approaches; 56% of these were to the very common parrotfish. Reactive pairs of spots were selected from four possible mantle locations and they were significantly likely to be directional towards fish, presumably as startle/warning, but not directed towards conspecifics as indicators of predator presence. Thus the evasion techniques of the cephalopods and their ability to produce different display patterns on the skin show how an intelligent animal modifies an otherwise simple decision about which pairs of spots to select from four possible mantle locations and whether to flee from potentially dangerous animals.     

Properties of synaptic transmission at the neuromuscular junction of the squid, Loligo opalescens

1. Spontaneous and evoked synaptic activity were recorded from the muscles of squid fin and mantle. These spontaneous synaptic potentials were large (up to 30 mV) and pleomorphic. Their amplitudes were not normally distributed, nor did they appear to be clustered in integral multiples of some "unit" event size. 2. Electrical stimulation of the nerve resulted in muscle twitches when the bath calcium concentration was a third normal or higher. The frequency of spontaneous synaptic events was unaffected by low calcium. 3. The large size of spontaneous events may mean that the synchronized release of only a few such "quanta" are sufficient to cause muscle action potentials and contraction. 4. The shapes of spontaneous events correlated poorly with their amplitudes, which is consistent with release from multiple synaptic sites with distinct properties.     

The organization of the muscle system of the causative agent of dicrocoeliosis,Dicrocoelium dendriticum

The musculature of parasitic flatworms plays a central role in locomotory movement, attachment to the host, and in the function of the digestive, reproductive, and excretory systems. We examine for the first time the muscle system of the flatworm Dicrocoelium dendriticum, a causative agent of the parasitic disease dicrocoeliosis, by use of fluorescently labeled phalloidin and confocal laser scanning microscopy. Somatic musculature of D. dendriticum consists of the circular, longitudinal, and diagonal muscles. The distribution of the muscle fibers in the body wall differed among the anterior, middle, and posterior body regions of the worm. The musculature of the attachment organs, the oral and ventral suckers, includes several types of muscles: the external equatorial and meridional muscles, internal circular and semicircular muscles, and radial muscles. Inside of the ventral sucker the diagonally located muscles were revealed and the supplementary u-shaped muscles were found adjoined to the base of the sucker from outside. The musculature of the internal organs composed of the excretory, reproductive, and digestive systems were characterized. Our results increase our knowledge of the morphology of trematodes and the arrangement of their muscle system.     

Fiber arrangement of the intestinal smooth muscle layers in ammocoetes of Lampetra japonica

The three-dimensional arrangement of the intestinal smooth muscle in the ammocoetes of the lamprey (Lampetra japonica) was examined by scanning electron microscopy (SEM) after removal of the intestinal mucosa. In cross section of the posterior midgut, its wall was composed of the parietal wall and the typhlosolar wall of the spiral fold, lining a horseshoe-shaped space, and had two distinct muscle layers. The fiber extensions of the muscle layers in the two parts of the wall were reversed; internal longitudinal and external circular in the parietal wall, but internal circular and external longitudinal in the typhlosolar wall. The positional exchange of the two layers occurred within the transitional area from the parietal wall to the typhlosolar wall, where an interlacing texture of longitudinal and circular braids of fibers was observed. Furthermore, the external fibers of the longitudinal braid extended successively into the circular braid until the longitudinal braid disappeared. However, any fibrous transition from the circular braid into the longitudinal braid in the typhlosolar wall was not clear in the present study. The internal location of the longitudinal layer at the parietal wall may be optimal for its main function of contracting the intestinal tract longitudinally. In addition, the external (to be precise, the internal to the hematopoietic tissue) longitudinal muscle layer in the typhlosolar wall may play an important role in saving and squeezing out blood into the cardinal intestinal vein by longitudinal contraction of the elongated adjacent hematopoietic tissue mass.