Malformations of the sacculus and the semicircular canals in spider morph pythons.

Spider morph ball pythons are a frequently-bred designer morph with striking alterations of the skin color pattern. We created high-resolution µCT-image series through the otic region of the skulls, used 3D-reconstruction software for rendering anatomical models, and compared the anatomy of the semicircular ducts, sacculus and ampullae of wildtype Python regius (ball python) with spider morph snakes. All spider morph snakes showed the wobble condition (i.e., twisting movements of the head, impaired locomotion, difficulty striking or constricting prey items). We describe the inner ear structures in wildtype and spider morph snakes and report a deviant morphology of semicircular canals, ampullae and sacculus in the latter. We also report about associated differences in the desmal skull bones of spider morph snakes, which were characterized by wider semicircular canals, ampullae widened and difficult to discern in µCT, a deformed crus communis, and a small sacculus with a highly deviant X-ray morphology as compared to wildtype individuals. We observed considerable intra- and interindividual variability of these features. This deviant morphology in spider morph snakes could easily be associated with an impairment of sense of equilibrium and the observed neurological wobble condition. Limitations in sample size prevent statistical analyses, but the anatomical evidence is strong enough to support an association between the wobble condition and a malformation of the inner ear structures. A link between artificially selected alterations in pattern and specific color design with neural-crest associated developmental malformations of the statoacoustic organ as known from other vertebrates is discussed.

Comparative Assessment of Computed Tomography and Magnetic Resonance Imaging of Spider Morph and Wild Type Ball Pythons (Python regius) for Evaluation of the Morphological Correlate of Wobble Syndrome.

There is general awareness of artificial selection and its potential implications on the health and welfare of animals. Despite growing popularity and increasing numbers of reptile breeds of atypical colour and pattern variants, only a few studies have investigated the appearance and causes of diseases associated with colour morphs. Ball pythons (Python regius) are among the most frequently bred reptiles and breeders have selected for a multitude of different colour and pattern morphs. Among those colour variants, the spider morph of the ball python is frequently associated with wobble syndrome. The aim of this study was to determine whether a morphological variant can be found and associated with the clinical occurrence of wobble syndrome in spider ball pythons, using computed tomography and magnetic resonance imaging as in-vivo diagnostic methods. Data from five spider and three wild type ball pythons was assessed and evaluated comparatively. We were able to identify distinctive structural differences in inner ear morphology in spider ball pythons, which were highly likely related to wobble syndrome. To our knowledge, this is the first report of these anomalies and provides a basis for further anatomical and genetic studies and discussion of the implications for animal welfare in reptile breeding.

Comparative and Functional Anatomy of the Ectothermic Sauropsid Heart.

The heart development, form, and functional specializations of chelonians, squamates, crocodilians, and birds characterize how diverse structure and specializations arise from similar foundations. This review aims to summarize the morphologic diversity of sauropsid hearts and present it in an integrative functional and phylogenetic context. Besides the detailed morphologic descriptions, the integrative view of function, evolution, and development will aid understanding of the surprising diversity of sauropsid hearts. This integrated perspective is a foundation that strengthens appreciation that the sauropsid hearts are the outcome of biological evolution; disease often is linked to arising mismatch between adaptations and modern environments.

Phylogeny and evolutionary history of the amniote egg.

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.

Morphology of the avian yolk sac.

The avian yolk sac is a multifunctional extraembryonic organ that serves not only as a site of nutrient (yolk) absorption, but also for early hemopoiesis, and formation of blood vessels. Although the yolk sac membrane being specialized to function as an extraembryonic absorptive organ, it is neither morphologically nor functionally part of the embryonic gut. Yolk absorption is by the phagocytic activity of the extraembryonic endoderm. I used cryohistology and resin embedding histology of complete developmental series of Japanese quail to document the development of the avian yolk sac and changes of the microscopic anatomy throughout development. This material is complemented by complete series of MRT-scans of live ostrich embryos from beginning of incubation through hatching. Considerable changes of size and shape of the yolk mass are documented and discussed as resulting from water flux from albumen to yolk associated with the biochemical activation of yolk sac proteins. During embryogenesis, the yolk sac endoderm forms villi that increase the absorptive surface and reach into the yolk ball. The histology of the absorptive epithelium is specialized for phagocytic absorption of yolk. During early developmental stages, the extraembryonic endoderm is single layered, but it eventually becomes several layers thick during later stages. The extraembryonic mesoderm forms an extensive layer of hematopoietic tissue; deep in this tissue lie the yolk sac vessels. During late stages of development, the erythropoietic tissue disappears, blood vessels are obliterated, and the yolk sac epithelium becomes apoptotic. Results are discussed in the light of the evolutionary history and phylogeny of the amniote egg.

Three genetically distinct ferlaviruses have varying effects on infected corn snakes (Pantherophis guttatus).

Ferlaviruses are important pathogens in snakes and other reptiles. They cause respiratory and neurological disease in infected animals and can cause severe disease outbreaks. Isolates from this genus can be divided into four genogroups-A, B, and C, as well as a more distantly related sister group, "tortoise". Sequences from large portions (5.3 kb) of the genomes of a variety of ferlavirus isolates from genogroups A, B, and C, including the genes coding the surface glycoproteins F and HN as well as the L protein were determined and compared. In silico analyses of the glycoproteins of genogroup A, B, and C isolates were carried out. Three isolates representing these three genogroups were used in transmission studies with corn snakes (Pantherophis guttatus), and clinical signs, gross and histopathology, electronmicroscopic changes in the lungs, and isolation of bacteria from the lungs were evaluated. Analysis of the sequences supported the previous categorization of ferlaviruses into four genogroups, and criteria for definition of ferlavirus genogroups and species were established based on sequence identities (80% resp. 90%). Analysis of the ferlavirus glycoprotein models showed parallels to corresponding regions of other paramyxoviruses. The transmission studies showed clear differences in the pathogenicities of the three virus isolates used. The genogroup B isolate was the most and the group A virus the least pathogenic. Reasons for these differences were not clear based on the differences in the putative structures of their respective glycoproteins, although e.g. residue and consequential structure variation of an extended cleavage site or changes in electrostatic charges at enzyme binding sites could play a role. The presence of bacteria in the lungs of the infected animals also clearly corresponded to increased pathogenicity. This study contributes to knowledge about the structure and phylogeny of ferlaviruses and lucidly demonstrates differences in pathogenicity between strains of different genogroups.

Morphological responses to feeding in ticks (Ixodes ricinus).

BACKGROUND: Ticks can survive long periods without feeding but, when feeding, ingest large quantities of blood, resulting in a more than 100-fold increase of body volume. We study morphological adaptations to changes in opisthosoma volume during feeding in the castor bean tick, Ixodes ricinus. We aim to understand the functional morphological features that accommodate enormous changes in volume changes. METHODS: Using light and electron microscopy, we compare the cuticle and epidermis of the alloscutum, the epithelium of the midgut diverticula, and the tracheae of adult female ticks when fasting, semi-engorged, and fully engorged. RESULTS: Our results add to an existing body of knowledge that the area of the epidermis increases by cellular differentiation, cellular hypertrophy, and changes in the shape of epithelial cells from pseudostratified to single layered prismatic in semi-engorged ticks, and to thin squamous epithelium in fully engorged ticks. We did not find evidence for cell proliferation. The midgut diverticula accommodate the volume increase by cellular hypertrophy and changes in cell shape. In fully engorged ticks, the epithelial cells of the midgut diverticula are stretched to an extremely thin, squamous epithelium. Changes in size and shape (and cell divisions) contribute to the accommodation of volume changes. Tracheae do not increase in size, but extend in length, thus following the volume changes of the opisthosoma in feeding ticks to secure oxygen supply to the internal organs. CONCLUSIONS: Changes of epithelial tissue configuration in the epidermis and the midgut diverticula are described as important components of the morphological response to feeding in ticks. We provide evidence for a previously unknown mechanism hosted in the endocuticle of the tracheae that allows the tracheae of castor bean ticks to expand when the body volume increases and the distance between the respiratory spiracle and the oxygen demanding tissue enlarges. This is the first report of expandable tracheae in arthropods.

Open data and digital morphology.

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

Histological and ultrastructural analysis of the respiratory tracheae of Galeodes granti (Chelicerata: Solifugae).

Solifuges lack oxygen-carrying proteins like hemocyanins found in other chelicerates. For conduction of gases, therefore, we hypothesize that the tracheal system is divided into convective and respiratory parts, the latter having intimate association with respiring cells, tissues and organs. This hypothesis is supported by studies of tracheae in other arthropods. We used light, scanning, and transmission electron microscopy to examine the tracheae of Galeodes granti (Chelicerata, Solifugae). We studied tracheae in cheliceral and leg muscles, midgut and midgut diverticula, the heart and the supra- and subesophageal ganglia of the central nervous system. In all tissues examined, the tracheae penetrate into tissues and can be found between cells. Terminal branches with diameter less than one micron were found embedded into cell bodies of respiring tissue, thus reducing the diffusion distance. We suggest that the terminal branches are specialized for respiratory function due to their thin intima, with reduced endocuticle and exocuticle. Comparison with other tracheate chelicerates suggests parallel evolution of a common tracheal pattern based on homologous cellular building materials within the chelicerates and arthropods.

Morphology of the tracheal system of camel spiders (Chelicerata: Solifugae) based on micro-CT and 3D-reconstruction in exemplar species from three families.

We studied the tracheal system of exemplar species representing three families of Solifugae Sundevall, 1833, i.e., Galeodes granti Pocock, 1903, Ammotrechula wasbaueri Muma, 1962 and Eremobates sp., using µCT-imaging and 3D-reconstruction. This is the first comparative study of the tracheal system of Solifugae in 85 years and the first using high-resolution nondestructive methods. The tracheal system was found to be structurally similar in all three species, with broad major tracheae predominantly in the prosoma as well as anastomoses (i.e., connections between tracheal branches from different stigmata) in the prosoma and opisthosoma. Differences among the three species were observed in the presence or absence of cheliceral air sacs, the number of tracheae supplying the heart, and the ramification of major tracheae in the opisthosoma. The structure of the tracheal system with its extensive branches and some anastomoses is assumed to aid rapid and efficient gas exchange in the respiratory tissues of these active predators. The large diameter of cheliceral tracheae (air sacs) of taxa with disproportionally heavier chelicerae suggests a role in weight reduction, enabling solifuges to reach greater speeds during predation. The air sacs may also permit more rapid and efficient gaseous exchange, necessary to operate the musculature of these structures, thereby improving their use for predation in an environment where prey is scarce.

Morphological specializations for fetal maintenance in viviparous vertebrates: An introduction and historical retrospective.

In many viviparous vertebrates, pregnant females sustain their developing embryos and provide them with nutrients by means of placentas and a diversity of other types of specializations. With this article, we introduce a virtual (online) issue of the Journal of Morphology that presents 12 recent papers on fetal maintenance in viviparous vertebrates. We also outline the history of research in this area and document the central role of morphology in helping to explain the function and evolution of specializations for fetal nutrition. This virtual issue of the Journal of Morphology is an outgrowth of a symposium held under auspices of the International Congress of Vertebrate Morphology. The included papers reflect a diversity of taxa, research methods, and biological issues. To celebrate the publication of this virtual issue of the Journal of Morphology, the publisher is making freely available to readers a number of other relevant papers published in the journal over the past 128 years.

Alligators and Crocodiles Have High Paracellular Absorption of Nutrients, But Differ in Digestive Morphology and Physiology.

Much of what is known about crocodilian nutrition and growth has come from animals propagated in captivity, but captive animals from the families Crocodilidae and Alligatoridae respond differently to similar diets. Since there are few comparative studies of crocodilian digestive physiology to help explain these differences, we investigated young Alligator mississippiensis and Crocodylus porosus in terms of (1) gross and microscopic morphology of the intestine, (2) activity of the membrane-bound digestive enzymes aminopeptidase-N, maltase, and sucrase, and (3) nutrient absorption by carrier-mediated and paracellular pathways. We also measured gut morphology of animals over a larger range of body sizes. The two species showed different allometry of length and mass of the gut, with A. mississippiensis having a steeper increase in intestinal mass with body size, and C. porosus having a steeper increase in intestinal length with body size. Both species showed similar patterns of magnification of the intestinal surface area, with decreasing magnification from the proximal to distal ends of the intestine. Although A. mississippiensis had significantly greater surface-area magnification overall, a compensating significant difference in gut length between species meant that total surface area of the intestine was not significantly different from that of C. porosus. The species differed in enzyme activities, with A. mississippiensis having significantly greater ability to digest carbohydrates relative to protein than did C. porosus. These differences in enzyme activity may help explain the differences in performance between the crocodilian families when on artificial diets. Both A. mississippiensis and C. porosus showed high absorption of 3-O methyl d-glucose (absorbed via both carrier-mediated and paracellular transport), as expected. Both species also showed surprisingly high levels of l-glucose-uptake (absorbed paracellularly), with fractional absorptions as high as those previously seen only in small birds and bats. Analyses of absorption rates suggested a relatively high proportional contribution of paracellular (i.e., non-mediated) uptake to total uptake of nutrients in both species. Because we measured juveniles, and most paracellular studies to date have been on adults, it is unclear whether high paracellular absorption is generally high within crocodilians or whether these high values are specific to juveniles.

Morphological respiratory diffusion capacity of the lungs of ball pythons (Python regius).

This study aims at a functional and morphological characterization of the lung of a boid snake. In particular, we were interested to see if the python's lungs are designed with excess capacity as compared to resting and working oxygen demands. Therefore, the morphological respiratory diffusion capacity of ball pythons (Python regius) was examined following a stereological, hierarchically nested approach. The volume of the respiratory exchange tissue was determined using computed tomography. Tissue compartments were quantified using stereological methods on light microscopic images. The tissue diffusion barrier for oxygen transport was characterized and measured using transmission electron micrographs. We found a significant negative correlation between body mass and the volume of respiratory tissue; the lungs of larger snakes had relatively less respiratory tissue. Therefore, mass-specific respiratory tissue was calculated to exclude effects of body mass. The volume of the lung that contains parenchyma was 11.9±5.0mm(3)g(-1). The volume fraction, i.e., the actual pulmonary exchange tissue per lung parenchyma, was 63.22±7.3%; the total respiratory surface was, on average, 0.214±0.129m(2); it was significantly negatively correlated to body mass, with larger snakes having proportionally smaller respiratory surfaces. For the air-blood barrier, a harmonic mean of 0.78±0.05µm was found, with the epithelial layer representing the thickest part of the barrier. Based on these findings, a median diffusion capacity of the tissue barrier ( [Formula: see text] ) of 0.69±0.38ml O(2)min(-1)mmHg(-1) was calculated. Based on published values for blood oxygen concentration, a total oxygen uptake capacity of 61.16mlO(2)min(-1)kg(-1) can be assumed. This value exceeds the maximum demand for oxygen in ball pythons by a factor of 12. We conclude that healthy individuals of P. regius possess a considerable spare capacity for tissue oxygen exchange.

Testing evolutionary hypotheses about the phylotypic period of zebrafish.

Vertebrate embryos pass through a period of morphological similarity, the phylotypic period. Since Haeckel's biogenetic law of recapitulation, proximate and ultimate evolutionary causes of such similarity of embryos were discussed. We test predictions about changes in phenotypic and genetic variances that were derived from three hypotheses about the evolutionary origin of the phylotypic stage, i.e. random, epigenetic effects, and stabilizing selection. The random hypothesis predicts increasing values for phenotypic variances and stable or increasing values for genetic variances; the epigenetic effects hypothesis predicts declining values for phenotypic variances but stable or increasing values of genetic variances, and the stabilizing selection predicts stable phenotypic variances but decreasing genetic variances. We studied zebrafish as a model species, because it can be bred in large numbers as necessary for a quantitative genetics breeding design. A half-sib breeding scheme provided estimates of additive genetic variances from 11 embryonic characters from 12 through to 24 hr after fertilization, i.e. before, during (15-19 hr), and after the phylotypic period. Because additive genetic variances are size dependent, we calculated narrow-sense heritabilities as a size independent gauge of genetic contributions to the phenotype. The results show declining phenotypic variances and stable heritabilities. In conclusion, we reject the random and the stabilizing selection hypotheses and favor ideas about epigenetic effects that constrain the early embryonic development. Additive genetic variance during the phylotypic stage makes it accessible for evolution, thus explaining in a simple and straightforward way why the phylotypic period differs among vertebrates in timing, duration, and morphologies.

Fuel switching and energy partitioning during the postprandial metabolic response in the ball python (Python regius).

Digestion, absorption and assimilation of the meal are active processes that require start-up energy before the energy contained in a meal can be utilized. The energetic costs associated with feeding (specific dynamic action, SDA) are high in sit-and-wait foraging snakes that tolerate long fasting periods. We used (13)C-labelled prey to partition between endogenous energy sources (i.e. snakes' own resources) and exogenous energy sources (i.e. prey). A linear mixing model was then applied to determine the portion of (13)C originating from the different sources. The snakes showed a normal and typical postprandial response. By four hours after feeding, the delta(13)C-values indicated fuel switching from endogenous to exogenous. From then on, fuel mixing continuously increased until, at 20 h after feeding, 75% of fuel was exogenous. Resource partitioning showed that throughout SDA, the amount of exogenous energy increased to approximately 60% of SDA, which was equivalent to approximately 4.5% of the energy contained in a meal.

Developmental plasticity, modularity, and heterochrony during the phylotypic stage of the zebra fish, Danio rerio.

We studied early embryonic development of zebra fish and tested if changes in the external raising conditions could elicit phenotypic changes during the phylotypic stage which, classically, is considered as a conserved embryonic stage. In particular, we tested for internal constraints, plasticity, and heterochrony during the early embryonic development. Our tested hypotheses predict (i) no change associated with developmental stability/internal constraints, (ii) change of the rate of development associated with developmental flexibility, and (iii) heterochronic disruption of developmental pattern associated with a modular organization of the embryo. We measured 14 traits of embryos raised in different conditions (temperature, salinity, oxygen concentration). The results of our study show that zebra fish embryos respond flexibly to changes in external parameters even during the conserved "phylotypic stage." It also showed that internal constraints canalize early development when exposed to moderate external challenges. Hypoxic conditions, however, elicited a heterochronic delay of the onset of the development of the Anlagen of the eye and the otic vesicle from the remaining embryo. Therefore, we concluded that the eye and the otic vesicle are modules that may develop, to a certain degree, independently of the rest of the embryo. Because these modules become recognizable only under specific raising conditions, we suggest that the modularization acts as buffering mechanism against extreme developmental deviations. Our results provide support to the idea that modularity is present during the phylotypic stage, but it is not effective under normal conditions.

Energy metabolism of Inuit sled dogs.

We explored how seasonal changes in temperature, exercise and food supply affected energy metabolism and heart rate of Inuit dogs in Greenland. Using open flow respirometry, doubly labeled water, and heart rate recording, we measured metabolic rates of the same dogs at two different locations: at one location the dogs were fed with high energy food throughout the year while at the other location they were fed with low energy food during summer. Our key questions were: is resting metabolic rate (RMR) increased during the winter season when dogs are working? Does feeding regime affect RMR during summer? What is the proportion of metabolic rate (MR) devoted to specific dynamic action (SDA), and what is the metabolic scope of working Inuit sled dogs? The Inuit dogs had an extremely wide thermoneutral zone extending down to -25 degrees C. Temperature changes between summer and winter did not affect RMR, thus summer fasting periods were defined as baseline RMR. Relative to this baseline, summer MR was upregulated in the group of dogs receiving low energy food, whereas heart rate was downregulated. However, during food digestion, both MR and HR were twice their respective baseline values. A continuously elevated MR was observed during winter. Because temperature effects were excluded and because there were also no effects of training, we attribute winter elevated MR to SDA because of the continuous food supply. Working MR during winter was 7.9 times the MR of resting dogs in winter, or 12.2 times baseline MR.

Muscle plasticity of Inuit sled dogs in Greenland.

This study examined flexible adjustments of skeletal muscle size, fiber structure, and capillarization in Inuit sled dogs responding to seasonal changes in temperature, exercise and food supply. Inuit dogs pull sleds in winter and are fed regularly throughout this working season. In summer, they remain chained to rocks without exercise, receiving food intermittently and often fasting for several days. We studied two dog teams in Northern Greenland (Qaanaaq) where dogs are still draught animals vital to Inuit hunters, and one dog team in Western Greenland (Qeqertarsuaq) where this traditional role has been lost. Northern Greenland dogs receive more and higher quality food than those in Western Greenland. We used ultrasonography for repeated muscle size measurements on the same individuals, and transmission electron microscopy on micro-biopsies for summer-winter comparisons of muscle histology, also within individuals. At both study sites, dogs' muscles were significantly thinner in summer than in winter - atrophy attributable to reduced fiber diameter. Sarcomeres from West Greenland dogs showed serious myofilament depletion and expansion of the sarcoplasmatic space between myofibrils during summer. At both study sites, summer samples showed fewer interfibrillar and subsarcolemmal mitochondria, and fewer lipid droplets between myofibrils, than did winter samples. In summer, capillary density was higher and inter-capillary distance smaller than in winter, but the capillary-to-fiber-ratio and number of capillaries associated with single myofibers were constant. Increased capillary density was probably a by-product of differential tissue responses to condition changes rather than a functional adaptation, because thinning of muscle fibers in summer was not accompanied by reduction in the capillary network. Thus, skeletal muscle of Inuit dogs responds flexibly to changes in functional demands. This flexibility is based on differential changes in functional components: mitochondrial numbers, lipid droplet size, and the number of contractile filaments all increase with increasing workload and food supply while the capillary network remains unchanged.

Functional morphology and patterns of blood flow in the heart of Python regius.

Brightness-modulated ultrasonography, continuous-wave Doppler, and pulsed-wave Doppler-echocardiography were used to analyze the functional morphology of the undisturbed heart of ball pythons. In particular, the action of the muscular ridge and the atrio-ventricular valves are key features to understand how patterns of blood flow emerge from structures directing blood into the various chambers of the heart. A step-by-step image analysis of echocardiographs shows that during ventricular diastole, the atrio-ventricular valves block the interventricular canals so that blood from the right atrium first fills the cavum venosum, and blood from the left atrium fills the cavum arteriosum. During diastole, blood from the cavum venosum crosses the muscular ridge into the cavum pulmonale. During middle to late systole the muscular ridge closes, thus prohibiting further blood flow into the cavum pulmonale. At the same time, the atrio-ventricular valves open the interventricular canal and allow blood from the cavum arteriosum to flow into the cavum venosum. In the late phase of ventricular systole, all blood from the cavum pulmonale is pressed into the pulmonary trunk; all blood from the cavum venosum is pressed into both aortas. Quantitative measures of blood flow volume showed that resting snakes bypass the pulmonary circulation and shunt about twice the blood volume into the systemic circulation as into the pulmonary circulation. When digesting, the oxygen demand of snakes increased tremendously. This is associated with shunting more blood into the pulmonary circulation. The results of this study allow the presentation of a detailed functional model of the python heart. They are also the basis for a functional hypothesis of how shunting is achieved. Further, it was shown that shunting is an active regulation process in response to changing demands of the organism (here, oxygen demand). Finally, the results of this study support earlier reports about a dual pressure circulation in Python regius.