synsacrum and pygostyle

Carina keel-shaped structures or ridges such as that on the breastbone of a bird used for flight muscle attachment Provided by the Springer Nature SharedIt content-sharing initiative. The pygostyle supports the tail feathers and muscles. What is the fate of the palatoquadrate cartilage in amphibians, Images and associated schematic diagrams of cleared and stained axial skeletons of chicken (Gallus gallus domesticus) embryos from developmental stages 28 to 30. From the early stages of 28 to 32, vertebrae are relatively uniform, with specific regions determined only by the presence of ribs (thoracic) or their adjacency to the pelvic girdle. the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in volume403,pages 155156 (2000)Cite this article. Modified sacral ribs provided bony connection of hind limbs to vertebral column to enhance weight-bearing. To obtain All embryos are accessioned in the Redpath Museum under lot number RM 084337. Postface downward or outward, 3. Costal processes increase in length until the 12th cervical, then diminish prior to the ribs of the thoracic series. Information and translations of synsacrum in the most comprehensive dictionary definitions resource on the web. From appendicular skeleton of bird - . Reptiles and birds ball-and-socket joint between first vertebra (atlas) and the single occipital condyle; specialized joint between atlas and axis allows for rotation In mammals, the sternum is divided into three segments (anterior to posterior), the manubrium, the sternebrae (ossified bony elements), and the xiphisternum and xiphoid cartilage. The first two non-specialized cervicals form a cartilaginous connection between their prezygapophyses and postzygapophyses. Omissions? Nearing the end of the series, the dorsal view of the vertebrae become more X-shaped with an increase in length of the transverse processes and a widening of the arch. F, stage 38 posterior schematic. Compare and contrast the sternum amongst the vertebrate classes. Vertebral identity is determined at the somite level within the segmental plate mesenchyme, whereas muscle rudiments do not possess such axial identity. Fusions within the axial column are represented by black shading. The most dorsal regions of the cartilaginous spinous processes of the 2nd to 5th thoracics are fused, with open spaces remaining underneath the fusions between the processes. At stage 39, ossification has begun in three cervicals and the fused thoracics. Pygostyle #6. Depending on the species, there may be consolidation of a series of cranial vertebrae, termed the notarium, or fusion of a series of caudal vertebrae, termed the synsacrum, which is a fusiform structure located on the underside of the pelvis. SH Arenas-Rodrguez The former two are also well preceded by fully chondrified segments prior to fusion, whereas the posterior lumbosacral fusion occurs only when those vertebrae chondrify. Blue is cartilage and dark purple represents ossified tissue within the images. -true parrots -cockatoos -new zealand parrots (hooked beak) Passerines? Once on land, gills would prove useless. The basal ornithuran Apsaravis has at least 12 cervicals and seven thoracic vertebrae (Clarke & Norell, 2002). The spinous processes are more defined and angled caudally, becoming more dorsally angled later in the series. Testing modularity and dissociation: the evolution of regional proportions in snakes. serve as sites of attachment for muscles and function as a protective Cartilage remains on the most distal ends of each zygapophysis. First two non-specialized cervical vertebrae are almost completely ossified except for the most caudal regions of the vertebrae. The most advanced ossification occurs in the atlas and axis, several stages before any other vertebral segment. The transition from non-avialian theropods to highly derived birds is accompanied with the loss of an elongate tail, fusion of posterior caudals into a pygostyle, a broadly fused synsacrum and, sometimes, a notarium. Thoracic ribs usually have two parts, a bony part (vertebral/costal rib) articulating with vertebrae, and a cartilage part called a costal cartilage (or sternal rib) which articulates with the sternum. The synsacrum is a unit formed by the fusion Tiny cartilaginous costal processes are present within the cervical series. The dorsalmost tip of the spinous process of the axis, as well as the ventralmost portion of the axis centrum, begin to ossify. Earth Sci. https://doi.org/10.1038/35003103. Light grey is cartilage and black represents ossified tissue in the diagrams. The relative delay of the 3rd and 4th cervicals correlates with the anteriorly delayed chondrification region. The delay in chondrification of the thoracic and posterior lumbosacrals may be associated with the later fusion of vertebrae in these regions with respect to the early fusion of the anterior lumbosacrals. Transverse processes cranial to the acetabulum thicken and angle cranially, caudally to the acetabulum, the processes begin to angle caudally. F. AXIAL SKELETON VERTEBRAE, RIBS, AND STERNUM. Additional free caudal vertebrae are likely present, but obscured by the left ischium and some soft tissue. (parseInt(navigator.appVersion) >= 3 )) || Zygapophysisprocess on the neural arch or spine; vertebral articulation C, stage 37 posterior schematic. More recent work of ossification sequences has been done for the skeletons of palaeognathous birds (Maxwell, 2008), quails (Nakamura et al., 2019) and frogs (Arenas-Rodrguez et al., 2018). C, stage 43 posterior schematic. The expression of Hox13 paralogs posteriorly acts to terminate tail elongation and neuronal proliferation (Economides et al., 2003). What are intervertebral discs? Lateral portion of atlas, and lateral and ventral regions of the axis, begin to ossify. Costal processes are elongate and ossified. 9). This forms a more extensive rigid structure than the pelvis of a mammal, fulfilling requirements for flight, locomotion and respiration. { The anterior region of seven fused synsacrals may correspond to the non-ornithuran pygostylian synsacrum of the same number of vertebrae. ADS 27 (Academy of Natural Sciences, Philadelphia, 1998). Birds also have a pygostyle, which is a flat bone Simplified vertebrate phylogeny with selected published Hox expression boundaries within the somitic mesoderm. Transverse processes rostral to the acetabulum angle more rostrally and those caudal to the acetabulum within the sacral series angle more caudally. The variation found within the avian axial column is directly related to the kinematics and biomechanics of the animal with increased or decreased mobility for flexibility and strength, respectively (Storer, 1982; James, 2009). Medial dorsal aspect of vertebral arch also remains cartilaginous, creating an interrupted ring representing the whole atlas. Name the vertebrates that possess a sternum. (((navigator.appName == "Netscape") && Graphing this reduction shows a steady trend in reduction from stage 33 through to hatchling, with one additional vertebral fusion into the posterior lumbosacrals posthatching (Fig. The first occurs between stages 32 and 33, when the 45 isolated segments fuse to 38. 21. Sues, H.-D. J. Vert. Internet Explorer). F, stage 44 posterior schematic. Diapophysis transverse process that supports the tuberculum of a rib Describe the structure of a typical vertebrae, including the following: centrum, neural arch, neural spine, hemal arch, hemal spine, chevron bone, neural (vertebral) canal, diapophyses, zygapophyses, prezygapophyses, and postzygapophyses. Distal ends of the ribs and their uncinate processes also remain cartilaginous. label numbers 1, 2, and 3 in figure A. Dens of the axis. Some Hox axial expression patterns are correlated with the developmental regionalization identified here, such as regions of advanced and delayed chondrification and vertebral fusions. The sclerotome surrounding the notochord ventrally will develop into vertebral bodies, while the dorsal portion surrounding the developing neural tube will form vertebral arches, under the influence of inducing substances. Gesell. PDF | On Jan 1, 2022, Masuood Ahmad John and others published Comparative Gross Anatomical Studies on The Free Caudal Vertebrae and Pygostyle of Kite, Crow and Pigeon Using Dermestid Beetle . By stage 40, the entire cervical and thoracic series has begun to ossify, as have the 1st through 7th lumbosacrals. Transverse processes of 8th through 11th lumbosacrals are fused. Define tuberculum and capitulum. The dorsalmost part of the arch, including the pre- and postzygapophyses and spinous processes, remain cartilaginous. Fusions are observed not just in the pygostyle, but also in the synsacrum and in the dorsal vertebrae anterior to the synsacrum, between the ribs as cross bridges called uncinate processes, and in the distal limbs. These specimens were cleared and stained for bone and cartilage according to the procedure of Dingerkus & Uhler (1977). These patterns to not follow the coarse axial regionalizations examined in the past and are worthy of future investigations. These fusions create individual functional units, or modules, within specific regions of the avian axial column. Instead, region-specific development of muscles is determined by the derivatives of the sclerotome and lateral plate mesoderm (Christ et al., 2000). Small ventral region of atlas is ossified. Birds have costal and sternal ribs, with uncinate processes for muscle attachment Embryol. Neural spine projection from neural arch The spinous processes appear as two parasagittal, cartilaginous protrusions. The transverse processes elongate where the rib articulations are present. for flight such as the keeled sternum for the attachment of large flight muscles and the fused vertebrae of the synsacrum and pygostyle. , Norell MA, Qiang JI, Keqin GAO. Four distinct lineages of underwater foraging birds exhibit an elongate, straight pygostyle, whereas aerial and terrestrial birds are characterized by a short, dorsally deflected pygostyle. if(MSFPhover) { MSFPnav1n=MSFPpreload("../_derived/home_cmp_expeditn010_hbtn.gif"); MSFPnav1h=MSFPpreload("../_derived/home_cmp_expeditn010_hbtn_a.gif"); } J Read More uncinate processes. Article The combination of active Hox genes specifies the region and identity of the vertebral elements, with alterations within the code leading to homeotic transformations and changes in segment identity (Kessel & Gruss, 1991). In the former two, the same vertebral segments that advance ossification also initiate fusions. Light grey is cartilage and black represents ossified tissue in the diagrams. Blue is cartilage and dark purple represents ossified tissue within the images. F, stage 32 posterior schematic. Ossification occurs through the series within the centra, but most are not complete. Delays in ossification are found to occur in conjunction with the formation of fusions. The axial skeleton, much like the rest of the from their reptile ancestors; fossil as well as some living reptiles also have The sternum of the bird is unique and further Sacral vertebra single, enlarged vertebra to articulate with pelvic girdle serves as an attachment site for tail feathers, which contribute to Silhouettes for Microraptor and Archaeopteryx are from Dececchi & Larsson (2013), Piscivoravis from www.phylopic.org, and Longipteryx and Hesperornis are new. The most delayed are the posterior two lumbosacral vertebrae. Hemal spine ventral projection from hemal arch The greater number of incorporated pygostyle and synsacrum vertebrae in the older specimen is likely due to progressive vertebral fusion (though individual variation cannot be discounted). For example, an additional subregion is identified within the lumbosacral region, between the 7th and 8th lumbosacrals that will be worthy of follow-up comparisons. The derived axial columns of mammals, crocodylians and birds have an additional lumbar region lacking ribs between the ribbed thoracic and sacral vertebrae. The bones of the tail, which articulate with the sacral vertebrae. G, stage 33 (day 78 of incubation) right lateral view. In both cases, the expanded synsacrum and pygostyle appear first. The spinous processes of the 2nd thoracic vertebrae through the 5th begin to fuse by stage 37. Reptiles ribs are short in neck, long in trunk G, stage 45 (day 1920 of incubation) right lateral view. Carinates have a massively enlarged sternum to support flight muscles. In birds, the adjacent bones of the pelvic girdle fuse into the os coxae, which fuses with the synsacrum, a fusion of multiple lumbosacral vertebrae. This structure can only be seen in birds. Contrast monocipital and bicipital ribs. Only by stage 45 have all remaining lumbosacral vertebrae ossified their centra in an anterior to posterior trend. Both these hesperornithiformes had 16 cervicals, suggesting a trend in cervical expansion in Ornithurae. Spinous process and postzygapophyses remain cartilaginous. , Zhou Z, Martin LD, Feduccia A. Hwang B, stage 40 right lateral schematic. In bony fishes, the centra ossify directly from mesenchyme surrounding the notochord. I, stage 33 posterior schematic. I, stage 35 posterior schematic. Nature (Nature) I, stage 42 posterior schematic. Hox A7 expression is along thoracics 27, which covers the zone of thoracic fusion (thoracics 25) into the notarium and the eventual fusion of thoracic 7 into the synsacrum. 14 April 2020, Receive 51 print issues and online access, Prices may be subject to local taxes which are calculated during checkout. And, upper part of the hyoid arch, a bony jaw brace in early fishes, became the stapes, located in a new part of the ear, the middle ear. Baptornis; Ca, Calciavis; Dr, Dromaius. synsacrum scapula dorsal vertebrae humerus ulna radius carpus ( carpometacarpus) metacarpus ( carpometacarpus) digits alula External anatomy (topography) of a typical bird: Beak Head Iris Five free caudal vertebrae are visible between the synsacrum and the pygostyle (Fig. The innominate bone and synsacrum are constructed Within Aves, notaria evolve independently at least 17 different times, occurring in Tinamiformes, Pelecaniformes, Threskiornithidae, Galliformes, Columbiformes and most Falconidae (Storer, 1982) and at least 12 independent times in oscine passerines (James, 2009). B, stage 37 right lateral schematic. Transverse processes also remain partially ossified. Vertebral counts of fossil birds and one modern bird were collected for Apsaravis ukhaanaClarke & Norell, 2001 (Clarke & Norell, 2002), Archaeopterygidae (Elzanowski, 2002), Baptornis advenusMarsh, 1877 (Martin & Tate, 1976), Calciavis grandeiNesbitt et al., 2016 (Nesbitt & Clarke, 2016), Confuciusornis sanctusHou et al., 1995 (Chiappe et al., 1999), Dromaius novaehollandiae (Latham, 1790) (Kumar & Singh, 2014), Hesperornis regalisMarsh, 1872a (Marsh, 1880), Ichthyornis dispar (Marsh, 1872b) (Clarke, 2004), Jeholornis primaZhou & Zhang, 2002a (Zhou & Zhang, 2003b), Longipteryx chaoyangensisZhang et al., 2001 (Zhang & Zhou, 2001), Microraptor guiXu et al., 2003 (Hwang et al., 2002; Pei et al., 2014), Orienantius ritteriLiu et al., 2019, Parapengornis eurycaudatusHu et al., 2015, Piscivoravis lii (Zhou et al., 2014). 43, 465482 (1888). 29. The bipartite fusion of the synsacrum in the chicken, with seven anterior synsacrals . Define autotomy. demonstrates how the skeletal system of the bird is well-suited. The beginnings of a crista spinosa synsacri appears as cartilage. Xu Sacral region three or more sacral vertebrae fuse to form a sacrum The most advanced postcervical regions are the middle thoracics, anterior lumbosacrals and caudals. Nearing the end of the series, intervertebral space between the arches increases and the transverse processes extend more laterally than caudally. The 8th through 10th caudals have also incompletely fused at this stage. 1). Anterior Hox A5 expression is present from cervicals 7 to 14, matching the posterior boundary of the anterior cervical delayed chondrification block and the advanced chondrification zone cervicals 714 that begins at stage 28. 21 December 2022, Communications Biology Mammal axial columns are highly regionalized and modular, having been mostly studied due to their conserved vertebral count. List the four main types of intercentra articulations. Caudal region chevron bones usually restricted to base of tail; coccyx Distal ends of the transverse processes and the proximal ends of the ribs remain cartilaginous. -canaries -finches -mynah birds The sternum consists of a plate lying ventral to the thoracic cavity and a median keel extending ventrally from it. The cartilaginous arches connecting the ala precetabularis have not fully fused but have thickened, and a cartilaginous crista spinosa synsacri has become more prominent. Meaning of synsacrum. Using a detailed description of the morphological development of the axial column in the model domestic chicken, Gallus gallus domesticus, we present a map of axial ossification based on discrete characters. Fusions of these Costal processes are ossified and extend in length throughout the series, but never extend past the length of their respective centrum. All of them extend more dorsally than caudally. The kidneys can be removed by cutting through the external iliac and ischiatic blood vessels on the lateral surfaces and carefully lifting them out of the dorsal wall of the synsacrum using a scalpel ( Fig. #4. 26. Ned. A, stage 31 (day 7 of incubation) right lateral view. Near the end of the series, small protrusions from the ventral side of the vertebral bodies appear as rib facets. Axial regionalization of the vertebrae is initially determined by Homeobox (Hox) gene expression (Burke et al., 1995). Light grey is cartilage and black represents ossified tissue in the diagrams. All embryos were fixed in 4% paraformaldehyde (PFA) prior to clearing and staining. Blue is cartilage and dark purple represents ossified tissue within the images. What is the pygostyle? Vertebral centra of the series are almost completely ossified. The axial column of birds varies significantly in number of individual vertebrae and anatomy across all taxa. Continuing through development, vertebrae become more specialized as cervical vertebrae begin to diverge with posteriorly angled spinous processes, prominent costal processes and laterally expanding transverse processes. Login . Currie, P. J. et al. The ribs were thought to be ventral processes of the axial skeleton and therefore to be derived from the sclerotomes; however, recently a dermomyotomal origin of the distal rib (the costal shaft) was suggested, with only the proximal parts (head and neck of the rib) being of sclerotomal origin. There is no trace of one region chondrifying before the other, nor the centrum or neural arch before each other. What is the dermatocranium? Lateral regions of atlas, and lateral and ventral region of axis, completely ossified. The innominate bones are fused with the synsacrum to a greater or lesser extent, according to species, forming an avian pelvis. Thoracic vertebrae 25 have fused cartilaginous spinous processes, but a full notarium is not present. This number increases at Ornithurae with the advent of higher cervical counts before reducing again in many Aves with the origin of notaria. Between 150 and 120 million years ago, . The innominate and In addition to problems of locomotion and support, the transition to land involved a large number of other challenges. Therefore, prior to understanding the evolutionary implications of the structure of the avian axial skeleton, we here describe and track its development in detail in order to assess anatomical evolution and the development of modularity. Our editors will review what youve submitted and determine whether to revise the article. Images and associated schematic diagrams of cleared and stained axial skeletons of chicken (Gallus gallus domesticus) embryos from developmental stages 34 to 36. You are using a browser version with limited support for CSS. The final adult axial segment number is only 24, as a result of the extensive fusions in the notarium, synsacrum and pygostyle. For permissions, please e-mail: [email protected], This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (, Origin of the pointed snout in Scolelepis and the mouth region in spionid polychaetes (Annelida: Spionidae), Morphological, ontogenetic, and molecular investigations of freshwater hypotrich ciliates from China revealed a new genus Heterodeviata gen. nov. (Protista: Ciliophora), and a novel limnetic population of Deviata multilineae, The search finds an end: the morphologically chimeric hysterocinetids belong to the subclass Hymenostomatia (Ciliophora: Oligohymenophorea), Untangling the threads: phylogenetic relationships of threadfins (Percomorphacea: Perciformes: Polynemidae). Dorsal region thoracic and lumbar vertebrae D, stage 44 (day 18 of incubation) right lateral view. Cervical vertebrae are uniform throughout the series with cartilaginous centra of equal length and width, neural arches and spinous processes angled caudally. A, stage 37 (day 11 of incubation) right lateral view. Fusion of the pygostyle is not complete, but space between centra is limited. What is their composition? of the posterior thoracic, lumbar, and sacral vertebrae. B, stage 28 right lateral schematic. Spinous processes are present on the left and right sides, present through the end of the series. Our study shows that the pattern and sequence of fusion and ossification during development may reflect the presence of independent modules as subsets within the typical regions of the avian axial column. The central section of the synsacrum is swollen to accommodate the glycogen body, an organ whose function is as yet unclear but which may be associated with balance. Light grey is cartilage and black represents ossified tissue within the diagrams. These maintain an only partially chondrified state until stage 44, separating the other fusing lumbosacral vertebrae from the caudals. -lumbar vertebrae are all fused together and form the synsacrum -caudal vertebrae stretch between the synsacrum and the pygostyle. This is a preview of subscription content, access via your institution. Both vertebrae are composed of cartilaginous centra, neural arches and spinous processes. There are no evolutionary intermediates, suggesting a rapid evolution to this reduced state (Rashid et al., 2014). The degree of ossification for each element of each vertebral segment was scored and presented in Supporting Information, Table S1. The origin of birds from their terrestrial antecedents was accompanied by a wholesale transformation of their skeleton as they transitioned from a terrestrial to aerial realm. Due to the transition of water to land, early tetrapod ribs were enlarged, giving rigidity to the trunk. Rhipidistian crossopterygians has vertebrae similar to those of earliest amphibians (Ichthyostegalia) with the notochord persistent, a neural arch, a large intercentrum, and small, paired pleurocentra. The 6th through 10th caudals within these stages also fuse incompletely during these stages. Thank you for visiting nature.com. In amphibians, the presence of zygapophyses limits dorsal/ventral bending. The early cartilaginous elements are relatively similar to one another and subsequently develop the characteristics unique to regions within the axial column and individual vertebrae. 8. Parker, W. K. Proc. As one of the major defining characteristics of Vertebrata, the axial column protects the spinal cord and dorsal aorta, provides attachment points for the axial musculature and is linked to body shape variation and a diverse suite of locomotory repertoires (Ward & Brainerd, 2007; Bergmann & Irschick, 2012; Ward & Mehta, 2014). Naturf. Nature 393, 753761 (1998). About Zoological Journal of the Linnean Society, https://doi.org/10.1093/zoolinnean/zlaa038, https://academic.oup.com/pages/standard-publication-reuse-rights, Receive exclusive offers and updates from Oxford Academic, Copyright 2023 The Linnean Society of London. I, stage 30 posterior schematic. Centrum principal part of a vertebra; below spinal cord; may consist of one or two elements. Jeholornis is similar with 49, with a few more caudal vertebrae (Zhou & Zhang, 2003b). Convergent evolution of a common pygostyle phenotype in diving birds suggests that this morphology is related to the mechanical demands of using the tail as Moreover, they do not cross particular regional boundaries, such as the cervicalthoracic, thoraciclumbosacral and lumbosacralcaudal boundaries. if(MSFPhover) { MSFPnav3n=MSFPpreload("_derived/new_page_2.htm_cmp_expeditn010_hbtn.gif"); MSFPnav3h=MSFPpreload("_derived/new_page_2.htm_cmp_expeditn010_hbtn_a.gif"); } Centra, arches, spinous processes and ventrally angled transverse processes are all present. Two principal lineages are derived from this common origin. , Chiappe LM, Zhang Y, Serrano FJ, Meng Q. Morin-Kensicki . Burke Theorize the phylogenetic origin of ribs. E, stage 29 right lateral schematic. This structure can only be seen in birds. The synsacrum is a unit formed by the fusion of the posterior thoracic, lumbar, and sacral vertebrae. In both cases, the expanded synsacrum and pygostyle appear first. show a close match of form and function. and JavaScript. Empty space between the arches decreases, and some fusion is present, hinting at development of the synsacrum. Modularity is formally described as the division of an organism into units that are products of a shared developmental origin, have hierarchical organization, interactions with similar units (Raff, 1996) and the ability to undergo developmental and evolutionary transformation (Bolker, 2000). Fusions along the axial column significantly reduce the mobility and increase stability within certain areas of the vertebral column, allowing for a various number of functional, behavioural and ecological implications (Mulder, 2001; James, 2009). Vereen. Axial patterning and development has also been described in zebrafish (Fisher & Halpern, 1999; Morin-Kensicki et al., 2002), alligators (Mansfield & Abzhanov, 2010), crocodiles (Bhmer et al., 2015a), turtles (Ohya et al., 2005) and snakes (Cohn & Tickle, 1999). The pygostyle is a fusion of the caudal vertebrae, which is found only in birds. Open Access , Charles C, Asher RJ, Gaunt SJ. MR 23. Anyone you share the following link with will be able to read this content: Sorry, a shareable link is not currently available for this article. H, stage 35 right lateral schematic. The vertebral column and ribs originate from somites, with the relationship between them being the subject of 150 years of investigation (Christ et al., 2000). Within Archosauria, these structural processes are highly disparate, with such structures and vertebral count significantly varying between birds, crocodylians and dinosaurs (Bhmer et al., 2015b; Scheyer et al., 2019). 25, 972986 (1998). This type of vertebra is called a rachitomous vertebra. Images and associated schematic diagrams of cleared and stained axial skeletons of chicken (Gallus gallus domesticus) embryos from developmental stages 43 to 45. Scale bar, 1 cm. ISSN 0028-0836 (print). C, stage 28 posterior schematic. Uncinate process #8. Arches and dorsal processes remain cartilaginous, with ossification beginning in the dorsalmost regions in vertebrae early in the thoracic series. Posterior to the synsacrum is a series of free tail (caudal) vertebrae and finally the pygostyle, which consists of several fused caudal vertebrae and supports the tail feathers. Include regional differentiation, heterocelous, synsacrum and pygostyle. Axial columns consist of small chondrifications in ring-like formations. A canary is not simply a small condor. Most flightless birds Costal processes are elongate and ossified, articulation remains cartilaginous with vertebrae. sternum is a prominent bone which juts out and provides increased surface area All embryos were then photographed in 100% glycerol with a Sony Rx1000. 6. created by the fusion of several vertebrae at the end of the column. Baumel, J. J. Adv. Internal lungs solved both of these problems by providing a specialized structure with a lot of small, branched air passageways which were effective in transfer of gases to and from the air and conservation of water.
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