Transverse Muscle Fibers Research Articles

Ultrastructure of the ventral pharynx in the interstitial annelid Questa paucibranchiata (Orbiniidae) and its phylogenetic significance

The small taxon Questa comprises only meiofaunal species and shows oligochaete-like adaptations in its reproductive biology. Formerly, it was classified as a separate polychaete group, Questidae, of uncertain affinities, but due to these oligochaete-like similarities, a relationship with Clitellata was discussed. However, other morphological phylogenetic analyses questioned such a relationship, providing evidence for common ancestry with Paraonidae and Orbiniidae. Molecular studies revealed that questids even form an in-group of Orbiniidae, which refutes a clitellate relationship without doubt. On the other hand, morphological evidence supporting this grouping is still considered weak. Among the external characters, the chaetation, comprising crenulated capillaries, hooks and furcate chaetae, might strengthen this hypothesis. One of the internal characters stated to support this grouping is the ventral pharynx, which as such, however, is widespread among polychaetes. This study is a first attempt to look for specific structures in this organ, which might be regarded as synapomorphic of Questa and Orbiniidae. The ultrastructural analysis revealed a ventral buccal organ composed of large interstitial cells and transverse muscle fibres and surrounded by an investing muscle system similar to that found in juvenile Orbiniidae. Thus, further morphological evidence for the current position is provided. Moreover, since this structure is known as a larval or juvenile character in Orbiniidae, this finding is also indicative for a progenetic origin of Questa, which might have occurred upon invading the interstitial environment.

The neuro-muscular system in cercaria with different patterns of locomotion

The neuro-muscular system (NMS) of cercariae with different swimming patterns was studied with immunocytochemical methods and confocal scanning laser microscopy. Specimens of the continuously swimming Cercaria parvicaudata, Maritrema subdolum and Himasthla elongata were compared with specimens of the intermittently swimming Cryptocotyle lingua and the attached Podocotyle atomon. The patterns of F-actin in the musculature, 5-HT immunoreactive (-IR), FMRFamide-IR neuronal elements, α-tubulin-IR elements in the nervous and sensory systems and DAPI-stained nuclei were investigated. The general plan of the NMS was similar in all cercariae studied. No major structural differences in the patterns of muscle fibres were observed. However, in the tail of C. lingua, transverse muscle fibres connecting the bands of longitudinal muscles were found. No major structural differences in the 5-HT- or FMRFamide-IR nervous systems were observed. The number of 5-HT-IR neurones in the cercarial bodies varied between 12 and 14. The number and distribution of the α-tubulin-IR processes on the cercarial bodies and tails differed from each other. The relation between the number and structure of the α-tubulin-IR processes and the host finding strategy of the cercariae is discussed. A detailed schematic picture of the NMS in the tails of C. lingua and M. subdolum is presented.

The arrangement and function of octopus arm musculature and connective tissue

The morphology of the musculature and connective tissues of the arms of Octopus bimaculoides was analyzed with light microscopy. We also studied O. briareus and O. digueti, which possess relatively more elongate and less elongate arms, respectively. The morphology of the arms was found to be remarkably uniform among species. The arms consist of a densely packed three-dimensional arrangement of muscle fibers and connective tissue fibers surrounding a central axial nerve cord. Three primary muscle fiber orientations were observed: 1) transverse muscle fibers oriented in planes perpendicular to the long axis of the arm; 2) longitudinal muscle fibers oriented parallel to the long axis; and 3) oblique muscle fibers arranged in helixes around the arm. The proportion of the arm cross section occupied by each of these muscle fiber groups (relative to the total cross sectional area of the musculature) remains constant along the length of the arm, even though the arm tapers from base to tip. A thin circular muscle layer wraps the arm musculature on the aboral side only. Much of this musculature has its origin and insertion on several robust connective tissue sheets including a layer surrounding the axial nerve cord and crossed-fiber connective tissue sheets located on the oral and the aboral sides of the arm. An additional thin layer of connective tissue wraps the arm musculature laterally and also serves as a site of origin and insertion of some of the muscle fibers. The fibers of the oral and aboral crossed-fiber connective tissue sheets are arranged oblique to the long axis of the arm with the same fiber angle as the oblique muscle layers that originate and insert on the sheets. The oblique muscle layers and the crossed-fiber connective tissue sheets thus form composite right- and left-handed helical fiber arrays. Analysis of arm morphology from the standpoint of biomechanics suggests that the transverse musculature is responsible for elongation of the arms, the longitudinal musculature is responsible for shortening, and the oblique muscle layers and associated connective tissues create torsion. Arm bending may involve unilateral contraction of longitudinal muscle bundles in combination with resistance to arm diameter increase due to contraction of the transverse musculature or passive stiffness of the arm tissues. The arms may also be bent by a combination of decrease in diameter due to contraction of the transverse musculature and maintenance of constant length on one side of the arm by unilateral activity of longitudinal muscle bundles. An increase in flexural stiffness of the arm may be achieved by cocontraction of the transverse and longitudinal muscle. Torsional stiffness may be increased by simultaneous contraction of both the right- and left-handed oblique muscle layers.

Fine structure of the pharyngeal apparatus of the pelagosphera larva in Phascolosoma agassizii (Sipuncula) and its phylogenetic significance

Sipuncula is a small taxon of worm-like marine organisms of still uncertain phylogenetic position. Si- punculans are characterized by an unsegmented body composed of a trunk into which the anterior part, the introvert, can be withdrawn. The group has been placed at various positions within Metazoa; currently, it is ei- ther seen as sister group of a clade comprising Mollusca and Annelida or as sister to each of these. An in-group position in either Mollusca or Annelida has usually been precluded till now due to the lack of so-called annelid or molluscan ''key-characters'' such as segmentation and chaetae or the radula. In the development of certain taxa the trochophore stage is followed by a planktonic larva, the pelagosphera, which might exhibit phylogenetically important structures. Among these is the buccal organ, which has been considered homologous either to the ventral pharyngeal organ present in many sedentary polychaetes or to the radular apparatus of molluscs. In the present paper, the ventral pharynx of the pelagosp- hera larva of Phascolosoma agassizii is investigated by transmission electron microscopy. The pharynx com- prises dorsolateral ciliary folds, a muscle bulb formed by transverse muscle fibres with large intercellular spaces, and an investing muscle. A tongue-like organ is lacking. These results show great structural correspondences to the ventral pharynx of polychaetes, especially to that of the flabelligerid Diplocirrus longisetosus. In contrast, there are no signs of structural similarities to the cor- responding structures of molluscs. Thus evidence in- creases that Sipuncula are closely related to annelids; moreover, an in-group position of Sipuncula within Annelida, as suggested by recent molecular studies, is not precluded by the present data. Instead these studies find additional support. Hence the lack of segmentation and chitinous chaetae in Sipuncula would be a second- ary rather than a primary situation, as has recently been shown for Echiura and Pogonophora.

Morphological studies for retrusive movement of the human adult tongue.

This study identified the anatomical and close functional relationship between the transverse lingual and superior pharyngeal constrictor muscle. Two en bloc samples (including the tongue and mid-pharyngeal wall) and four whole tongues were obtained from adult human cadavers. We found that fibers of the superior pharyngeal constrictor muscle connected with fibers of the transverse lingual muscle, forming a ring of muscle at the base of the tongue. The average diameters of the transverse muscle fibers increased in size gradually as they approached the base of the tongue. Distribution of the muscle spindles in the transverse lingual muscle and the genioglossus muscle also increased as they reached posteriorly near the base of the tongue. These findings suggest that a ring of muscle formed by the postero-inferior portion of the transverse lingual muscle and the superior pharyngeal constrictor may be largely responsible for the retrusive movement of the tongue and the constrictive movement of the pharyngeal cavity as an antagonist of the genioglossus muscle.

Difficult Laryngoscopy Due to the Abnormal Palatoglossal Arch

To the Editor:—The palatoglossal arch curves downward and forward from the soft palate to the tongue and forms the lateral part of the isthmus faucium with the palatopharyngeal arch. We experienced difficult laryngoscopy in a patient with an anatomically abnormal palatoglossal arch.A 15-yr-old boy with mental retardation caused by glycogen-storage disease with frequent hypoglycemic episodes was scheduled for dental treatment during general anesthesia. Premedication consisted of scopolamine and pentazocine. Anesthesia was induced with intravenous midazolam and inhalation of nitrous oxide, oxygen, and sevoflurane. Vecuronium was administered intravenously to facilitate endotracheal intubation. A Macintosh blade was inserted into his mouth and advanced between the right molars and the right side of the tongue. The tongue was about to be displaced to the left to visualize the larynx when the laryngoscopist noticed that it was impossible to displace the base of the tongue with the Macintosh blade. She observed the anatomic relation between the tongue and the pharynx to clarify the reason why the tongue could not be displaced and laryngoscopy was difficult. Her observation of the base of the tongue revealed that the right palatoglossal arch was attached to the dorsal part of the tongue, not to the lateral part (fig. 1). The palatoglossal arch contains the palatoglossus muscle, which originates in the oral surface of the palatine aponeurosis, extends forward, downward, and laterally in front of the palatine tonsil, and enters the lateral part of the tongue, passing deeply and transversely through the tongue with intrinsic transverse muscle fibers. 1The muscle elevates the posterior part of the tongue and pulls down the soft palate, thus constricting the isthmus of fauces and closing off the oral cavity from the oropharynx. 2During ordinary laryngoscopy with the Macintosh blade, the blade was advanced along the right lingual edge toward the right molars and the base of the tongue, then into the pharynx, while the tongue is displaced to the left. However, in this patient, a laryngoscopist could not displace the tongue to the left while advancing the blade along the right lingual edge because the right palatoglossal arch was attached to the dorsal part of the tongue. This anatomic abnormality prevented the displacement of the tongue to the left, which lead to difficult laryngoscopy using the Macintosh blade. The laryngoscopist tried a different method of laryngoscopy without displacement of the tongue to the left. She inserted the Macintosh blade into the open mouth and advanced it forward along the center of the dorsum of the tongue and pushed the base of the tongue upward without displacing it to the left. This maneuver made arytenoid visualization possible. The palatopharyngeal arch, which was located behind the palatoglossal arch, was normal, and there was no abnormality in the relation between these arches. In general, the so-called Mallampati test is available to predict difficult laryngoscopy. 3However, preoperative intraoral examination was difficult in this patient because of mental retardation. Based on our experience, we propose that the abnormal palatoglossal arch should be described as one of the anatomic factors that make laryngoscopy difficult to perform.

Neuromuscular system of the flexible arm of the octopus: physiological characterization.

The octopus arm is an outstanding example of an efficient boneless and highly flexible appendage. We have begun characterizing the neuromuscular system of the octopus arm in both innervated muscle preparations and dissociated muscle cells. Functionally antagonistic longitudinal and transverse muscle fibers showed no differences in membrane properties and mode of innervation. The muscle cells are excitable but have a broad range of linear membrane properties. They are electrotonically very compact so that localized synaptic inputs can control the membrane potential of the entire muscle cell. Three distinct excitatory neuronal inputs to each arm muscle cell were identified; their reversal potentials were extrapolated to be about -10 mV. These appear to be cholinergic as they are blocked by hexamethonium, D-tubocurarine, and atropine. Two inputs have low quantal amplitude (1-7 mV) and slow rise times (4-15 ms), whereas the third has a large size (5-25 mV) and fast rise time (2-4 ms). This large synaptic input is most likely due to exceptionally large quantal events. The probability of release is rather low, suggesting a stochastic activation of muscle cells. All inputs demonstrated a modest activity-dependent plasticity typical of fast neuromuscular systems. The pre- and postsynaptic properties suggest a rather direct relation between neuronal activity and muscle action. The lack of significant electrical coupling between muscle fibers and the indications for the small size of the motor units suggest that the neuromuscular system of the octopus arm has evolved to ensure a high level of precise localization in the neural control of arm function.

Muscle development in squid: Ultrastructural differentiation of a specialized muscle fiber type.

The ultrastructural differentiation of two muscle fiber types of the squid Sepioteuthis lessoniana was correlated with development of prey-capture behavior. Transmission electron microscopy was used to document the differentiation of the fast-contracting cross-striated muscle cells of the tentacles and the obliquely striated muscle cells of the arms of specimens sampled at one week intervals from hatching to 5 weeks. By using high-speed video recordings, the ultrastructural differentiation was correlated with changes in prey-capture behavior that occur during development and growth. The ultrastructural analysis focused on the muscle cells of the transverse muscle of the tentacles and the transverse muscle of the arms. For the first 2 weeks after hatching, the tentacle transverse muscle fibers do not show the adult ultrastructure and are indistinguishable from the obliquely striated fibers of the transverse muscle of the arms. Transverse striation of the tentacle muscle cells appears at approximately three weeks and adult ultrastructure is present by 4-5 weeks after hatching. The high-speed video recordings show correlated behavioral changes. During the first 2-3 weeks after hatching, the animals use a different prey-capture mode from the adults; they jet forward and capture the prey with splayed arms and tentacles rather than employing the rapid tentacular strike. © 1996 Wiley-Liss, Inc.

Internal morphology of the gill of a loricariid fish,Hypostomus plecostomus: arterio-arterial vasculature and muscle organization

The structural organization of the interbranchial septum of the gill arch of the air-breathing loricariid fish Hypostomus plecostomus was examined using light and electron microscopy. In the middle of the interbranchial septum, an extensive interconnection was found between the afferent primary arteries from successive and opposing primary lamellae. The blood circulates among numerous trabeculae consisting of connective tissue, smooth muscle cells, and collagen fibres. A sheet of smooth muscle cells is localized at the borders of these interconnected primary arteries and joins the cartilage rod from one primary lamella to the adjacent one on the same hemibranch. The adductor muscles are restricted to the distal end of the interbranchial septum and consist of transverse and oblique striated muscle fibres fixed to the cartilage rod from the primary lamella of opposite hemibranchs. The arrangement of these muscle fibres suggests a double movement of adduction: approximation of the tips of the primary lamellae of opposing hemibranchs and reduction of the space between adjacent primary lamellae of the same hemibranch. The action of both smooth and striated muscles reduces the interconnecting vascular septal space between the primary arteries, which may allow fine adjustment of vascular perfusion of the distal part of the filaments as an adaptation for better blood flow under hypoxic conditions.

A histological and chemical study of the cerata of the opisthobranch molluscTethys fimbria

Tethys fimbria is a poorly studied nudibranch that, recently, has been shown to possess large amounts of unusual prostaglandin (PG) derivatives. The cerata ofT. fimbria collected in the Bay of Naples (Italy) between November 1990 and January 1991 were submitted to histological analysis. This revealed the presence of both longitudinal and transverse smooth muscle fibres together with multicellular defensive glands, responsible for the typical spontaneous contractions of the cerata and for the extrusion of a hyaline secretion, respectively. A role forT. fimbria cerata in mollusc defense behaviour is suggested. Chemical analysis of the cerata led to the isolation of high levels of prostaglandin free acids (PGs) and of PG-1,15-lactones of the E, A and F series, whereas the secretion contained only PG-1,15-lactones of the E and A series. The latter were found to be toxic to mosquito fish at concentrations ranging between 1 and 10 µg ml−1, whereas PG free acids and PG-1,15-lactones of the F series were not toxic. The possibility that PGE- and PGA-1.15-lactones areT. fimbria defense allomones is discussed.

The fin musculature of cuttlefish and squid (Mollusca, Cephalopoda): morphology and mechanics

The lateral fins of cuttlefish and squid consist of a tightly packed three‐dimensional array of musculature that lacks bony skeletal support or fluid‐filled cavities for hydrostatic skeletal support. During swimming and manoeuvring, the fins are bent upward and downward in undulatory waves. The fin musculature is arranged in three mutually perpendicular planes. Transverse muscle bundles extend parallel to the fin surface from the base of the fin to the fin margin. Dorso‐ventral muscle bundles extend from dorsal and ventral connective tissue fasciae to a median connective tissue fascia. A layer of longitudinal muscle bundles is situated adjacent to both the dorsal and ventral surface of the median fascia. The muscle fibres are obliquely striated and include a core of mitochondria. A zone of muscle fibres with a more extensive core of mitochondria is present in both the dorsal and the ventral transverse muscle bundles. It is hypothesized that these muscle masses include two fibre types with different aerobic capacity. A network of connective tissue fibres is present in the transverse and dorso‐ventral muscle masses. These fibres, probably collagen, are oriented at 45 to the long axes of the transverse and dorsoventral muscle fibres in transverse planes.A biomechanicayl analysis of the morphology suggests that support for fin movements is provided by simultaneous contractile activity of muscles of specific orientations in a manner similar to that proposed for other ‘muscular‐hydrostats’. The musculature therefore provides both the force and support for movement. Connective tissue fibres may aid in providing support and may also serve for elastic energy storage.

The Tapeworm Spiniloculus-Mavensis (Tetraphyllidea, Onchobothriidae) From the Brownbanded Bambooshark in Australia

Sixteen specimens of the tapeworm Spiniloculus mavensis were collected from the spiral valve of the brownbanded bambooshark in Australia. These specimens were examined with both light and scanning electron microscopy (SEM) and were compared with specimens of Acanthobothrium lineatum. The scolices of 50% of the specimens of S. mavensis were spined. The concept of a loculus is discussed; it was determined that each of the bothridia of S. mavensis possesses three loculi distinguished by two types of costae. The anterior costa is composed of radial muscle fibres similar to the costae in other onchobothriids such as A. lineatum; the posterior costa consists of transverse muscle fibres with little external representation and is therefore difficult to see with SEM. It is suggested that this construction accounts for the disagreement among previous authors with respect to the number of post-hook loculi. The specific description of S. mavensis is supplemented and the description of Spiniloculus is emended.

Anatomy and ultrastructure of the ventral pharyngeal organs of Saccocirrus(Saccocirridae) and Protodriloides(Protodriloidae fam. n.) with remarks on the phylogenetic relationships within the Protodrilida (Annelida: Polychaeta)

General anatomy and ultrastructure of the ventral pharyngeal organs were investigated in Succocirrus krusudensis, Protodriloides chaetifer and P. symbioticus. Succocirrus papillocercus, a species without a ventral pharyngeal organ, was included for comparison. The two genera show homologous similarities in their pharynges: bulbus muscle composed of transverse muscle fibres and interstitial cells, those cells with small cell bodies and processes containing prominent tonofilaments which are orientated dorsoventrally and connect the bulbus epithelium with the investing muscle; bulbus muscle fibres circomyarian with nuclei and mitochondria located basally, investing muscle also with interstitial cells, which lack tonofilaments; oesophagus surrounded by gland cells opening into the pharynx. Most likely, a ventral pharynx with these characters was already present in the stem species of Saccocirridae, Protodriloidae fam. n. and Protodrilidae and evolutionary processes led to several changes: a tongue‐like organ with a prominent tip and supporting elements is a synapomorphic character of Protodrilus and Succocirrus; the pharyngeal organ is reduced to stomodeal pouches and salivary glands in S. papillocercus and completely lost in Asromus raenioides. These results and data from previous studies are summarized in a tentative phylogenetic dendrogram and allow the introduction of a new family, Protodriloidae.

Anatomy and ultrastructure of ventral pharyngeal organs and their phylogenetic importance in Polychaeta (Annelida)

Transmission electron microscopic studies were carried out on the ventral pharyngeal organs in Ctenodrilus serratus and Scoloplos armiger. The pharyngeal organs are composed of a muscle bulbus and a tongue-like organ. In both species the muscle bulbus consists of transverse muscle fibres and interstitial cells with voluminous cell bodies and dorsoventral tonofilaments; the investing muscle runs into the tongue-like organ; the nuclei of the investing muscle fibres are located in caudal bulges; salivary glands are not present, but numerous gland cells occur in the bulbus epithelium. The tongue-like organ, however, is formed by lateral folds (C. serratus) or a bridge-like structure (S. armiger). The specific structure of the bulbus muscle is probably a homologous characteristic also occurring in several other polychaete families. The phylogenetic importance of this ventral pharynx is discussed and a hypothesis is suggested to explain the differentiation of certain other ventral pharyngeal organs from this probably primitive type.

THE STRUCTURE OF THE SECONDARY GILLS OF SIPHONARIA CAPENSIS (GASTROPODA: PULMONATA)

The secondary gill of the pulmonate limpet Siphonaria capensis is located in the dorsal portion of the mantle cavity. Each lamella of the gill is triangular in shape and bears ciliary tufts which have a density of about 400/mm2. The free ends of some of the cilia are enlarged biconcave discs. Each gill lamella is covered by a single layer of cuboidal (4×4 μm) epithelium on each side, separated by a central haemocoelic space. Ciliated and mucus secreting cells are interspersed amongst the epithelial cells. The haemocoelic space is spanned at intervals by trabeculae containing longitudinal and transverse muscle fibres.

Observations on the histology and histochemistry ofPenetrocephalus plerocercoid (Pseudophyllidea: Cestoda)

The histology of plerocercoid ofPenetrocephalus sp. reveal that the body surface consists of tegument, basement membrane, epidermal longitudinal musculature, parenchymatic longitudinal musculature, transverse and dorsoventral muscle fibres. Three types of glands could be recognized from the scolex of the plerocercoid. The musculature of the plerocercoid consists of glycogen, acid, sulfated, neutral and carboxylated mucopolysac-charides: basic proteins containing tyrosine, S-H and S-S groups, protein bound amino groups, sulfhydril groups, glycoprotein and lipid.

Problems associated with the morphometric measurement of transverse skeletal muscle fibers: I. Analysis of frozen sections.

The aim of this study was to evaluate the effects of a number of reported procedures which precede freezing on the cross sectional area of skeletal muscle fibers. Probable sources of variation were recognised as: Within blocks, same animal, same procedure. Variability in the size of muscle fibers from area-based measurements was determined for the anterior head of the biceps femoris muscle of the rat. Fibers were measured within areas selected at random from sections taken from the same block of tissue. Variance within and between sections was subsequently established. Between blocks from rats having undergone the same procedure. Variability in the size of muscle fibers measured within areas selected at random from sections taken from different blocks of tissue within the same animal. Between rats, same or equivalent procedure. Fibers were measured from randomly selected areas within sections taken from blocks of tissue derived from different animals. The muscles were subjected to the same or equivalent treatment prior to freezing. Between procedures. Restraining muscles prior to and during freezing did not significantly affect the results except the procedure involving holding the muscle between two pairs of forceps. However, the variability was high within and between sections and blocks from the same muscle. It is postulated that the variability is of technical rather than biological origin and may be the result of small focal contractions which occur along the fibers during freezing.

The functional morphology of the musculature of squid (Loliginidae) arms and tentacles.

The arms and tentacles of squid (Family Loliginidae: Sepioteuthis sepioidea (Blainville), Loligo pealei (LeSueur), Loligo plei (Blainville), Loliguncula brevis (Blainville)) do not possess the hardened skeletal elements or fluid-filled cavities that typically provide skeletal support in other animals. Instead, these appendages are made up almost entirely of muscle. It is suggested here that the musculature serves as both the effector of movement and as the skeletal support system itself. High-speed movie recordings were used to observe prey capture by loliginid squid. Extension of the tentacles (1 pair) during prey capture is probably brought about by contraction of transverse muscle fibers and circular muscle fibers. Contraction of longitudinal muscle fibers causes retraction of the tentacles. Torsion of the tentacles during extension may be the result of contraction of muscle fibers arranged in a helical array. The inextensible but manipulative arms (4 pairs) may utilize a transverse muscle mass to resist the longitudinal compression caused by contraction of the longitudinal muscles which bend the arms. A composite connective tissue/muscle helical fiber array may twist the arms.

Structure of the membranous trachea in children.

The structure of the membranous wall of the trachea was studied in 560 tracheas from children aged 24 weeks of gestation to 10 years. The transverse muscle fibres are uniformly arranged. This is in marked contrast to the longitudinal smooth muscle which varies throughout the entire tracheal length. In 16% of tracheas, the longitudinal muscle was absent, while in the other 84% its distribution was variable with the muscle being present in the lower half of the trachea in approximately 70% of cases. It is suggested that the longitudinal muscle may be active in preserving the stability of the tracheal wall, preventing tracheal collapse and obliteration of the lumen, particularly in the intrathoracic part of the trachea.

Studies of Comparative Ultrastructure of Snake Cardiac Tissue

The ultrastructure of mammalian cardiac muscle has been the subject of numerous investigations attempting to correlate structure with function (Fawcett and McNutt, 1969). As a result there is a wealth of comparative data available for mammalian cardiac muscle. Less information is available concerning the comparative ultrastructure of cardiac tissue in other vertebrate classes. In an attempt to gain more information concerning the structure-function correlations, Sommer and Johnson (1969) compared the cardiac muscle of representative species of various classes of vertebrates and noted differences in structure and concentration of transverse tubules, sarcoplasmic reticulum and muscle fibers. The comparative ultrastructure of reptilian cardiac tissue has received less attention than most other classes of vertebrates and data specifically dealing with snakes are rare. Leak (1967) reported the only detailed ultrastructural study of the cardiac tissue of a snake, the boa constrictor in the family Boidae. His work depicts several distinct features present in snake cardiac muscle which are not found in other vertebrate cardiac tissue.