Diploblastic animals may have cell types that serve multiple functions, such as epitheliomuscular cells, which serve as a covering as well as contractile cells. Further subdivision of animals with three germ layers triploblasts results in the separation of animals that may develop an internal body cavity derived from mesoderm, called a coelom , and those that do not.
This epithelial cell-lined coelomic cavity , usually filled with fluid, lies between the visceral organs and the body wall. It houses many organs such as the digestive, urinary, and reproductive systems, the heart and lungs, and also contains the major arteries and veins of the circulatory system.
In mammals, the body cavity is divided into the thoracic cavity, which houses the heart and lungs, and the abdominal cavity, which houses the digestive organs. In the thoracic cavity further subdivision produces the pleural cavity, which provides space for the lungs to expand during breathing, and the pericardial cavity, which provides room for movements of the heart.
The evolution of the coelom is associated with many functional advantages. For example, the coelom provides cushioning and shock absorption for the major organ systems that it encloses. In addition, organs housed within the coelom can grow and move freely, which promotes optimal organ development and placement. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility, promoting improved animal motility.
Triploblasts that do not develop a coelom are called acoelomates , and their mesoderm region is completely filled with tissue, although they do still have a gut cavity. Examples of acoelomates include animals in the phylum Platyhelminthes, also known as flatworms. Animals with a true coelom are called eucoelomates or coelomates Figure.
In such cases, a true coelom arises entirely within the mesoderm germ layer and is lined by an epithelial membrane. This membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some freedom of movement.
Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. A third group of triploblasts has a slightly different coelom lined partly by mesoderm and partly by endoderm. The phylum Nematoda roundworms is an example of a pseudocoelomate. True coelomates can be further characterized based on other features of their early embryological development. Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in the origin of the mouth.
When the primitive gut forms, the opening that first connects the gut cavity to the outside of the embryo is called the blastopore. Most animals have openings at both ends of the gut: mouth at one end and anus at the other.
One of these openings will develop at or near the site of the blastopore. In Protostomes "mouth first" , the mouth develops at the blastopore Figure.
In Deuterostomes "mouth second" , the mouth develops at the other end of the gut Figure and the anus develops at the site of the blastopore. Protostomes include arthropods, mollusks, and annelids. Recent evidence has challenged this simple view of the relationship between the location of the blastopore and the formation of the mouth, however, and the theory remains under debate. Nevertheless, these details of mouth and anus formation reflect general differences in the organization of protostome and deuterostome embryos, which are also expressed in other developmental features.
One of these differences between protostomes and deuterostomes is the method of coelom formation, beginning from the gastrula stage. Since body cavity formation tends to accompany the formation of the mesoderm, the mesoderm of protostomes and deuterostomes forms differently. The coelom of most protostomes is formed through a process called schizocoely. The mesoderm in these organisms is usually the product of specific blastomeres , which migrate into the interior of the embryo and form two clumps of mesodermal tissue.
Within each clump, cavities develop and merge to form the hollow opening of the coelom. Deuterostomes differ in that their coelom forms through a process called enterocoely. Here, the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse and expand to fill the space between the gut and the body wall, giving rise to the coelom.
Once the morphogenetic center at the future oral pole in Punctatus was established, it may have formed oral ruffles repeatedly and elongated its conical body Figure 4. As the gut endoderm in Punctatus did not line the epidermis, it is only the oral region in which the ectoderm and endoderm meet.
With this body plan, unlike extant cnidarians, it would have been difficult to reproduce by strobilation. Asexual reproduction such as budding, body fission, and strobilation is common in extant cnidarians. Although fossil remains from the Kuanchunpu Formation have revealed large amounts of embryonic fossils, no trace of budding or body fission in the Punctatus conical body fossils has been found.
This suggests that in Punctatus sexual reproduction was the standard mode of reproduction. Egg is polarized with high concentration of maternal factors at a pole, most likely the animal pole, as maternal factors tend to associate with the germinal vesicle. Depending on maternal factors, a growth zone is established at the pole where gastrulation takes place, and then the oral ruffle forms with a basic penta-radial pattern.
The circular growth zone at the oral region retains its morphogenetic ability throughout life and periodically renews the oral ruffle. Old oral ruffles move to the periphery and transform into annular fringes of the column, driving the growth of the animal body. With this growth pattern, the embryonic body is retained at the tip of the body. One major drawback to the present reconstruction of these fossils without tentacles and with a very small gut throughout life is a potential method of feeding.
One possibility is that Punctatus acquired nutrients from symbionts, and such systems are often observed in modern animals. As a body plan comparable to Punctatus is not found in extant animals, Punctatus may have depended on another method of nutrition. Modern Protohydra lacks tentacles and has a spindle or club-shaped body when relaxed.
It is slightly smaller than Punctatus but highly elastic and moves in a screwing motion to find prey [37]. A similar predatory behavior is unlikely for Punctatus as its undulated body and the lack of a holding structure does not fit with such behavior. A fully occupied body cavity in a minute body of Punctatus could function as a reservoir for nutrients, as well as a hydrostatic skeleton and hydrodynamic system to drive the oral ruffle.
Among the huge variety of cnidarians, only a small number of representatives have been fully studied. Therefore it is not easy to conclude whether or not the above-mentioned characters are enough erect a new taxon separate from Cnidaria for Punctatus. Placozoans are very simple discoidal eumetazoans that have upper ectoderm-like and bottom endoderm-like epithelia with internal multinucleate fiber cells [39].
The two different types of the epithelia could theoretically indicate the cnidarian affinity of placozoans, even though they have no internal gut. In fact, placozoans were once regarded as secondarily simplified cnidarians [40]. Given that simple body forms make differences inconspicuous, we are in favor of the theory that Punctatus was a stem member of the diploblastic eumetazoans that potentially comprised an independent clade from Cnidaria due to the following reasons.
Firstly, Punctatus grew through a unique terminal addition, retaining a gastrula-like body pattern that separated the epidermis from the small gut with a body cavity, making it distinct from Porifera, Ctenophora, and Cnidaria Figure 5. Secondly, such development logically requires only an egg axis, and would have freed the animal from any bias of asymmetry.
Finally, Punctatus lacks many of the basic cnidarian characters. The developmental pattern of Punctatus was similar to that of cnidarian actiniarians sea anemones , but was generally distinct from phylum Cnidaria by the following features; no partitions of the gut, large body cavity, and benthic hatchlings. Blue broken lines in cnidarian and Punctatus larvae represent respective transverse sections.
Porifera pattern after [41] , [42] and Ctenophora pattern after [36] , [43]. All fossil specimens were collected from Kua — [3] in the Kuanchuanpu Formation of the Shizhonggou Section at Ningqiang, Shaanxi, China between and As the study area is not listed as the first or second class protection area of paleontological resources, no specific permits were required for the described field surveys The Regulations of Protection of Paleontological Resources, Chapter Two, P.
The number of the specimens studied was greater than 10, The extraction of the fossils from rock samples, basic observation by SEM, and micro-CT analyses to observe internal structures were the same as those in [33]. The fossils that we relied on in the interpretation of the embryonic development were selected based on their intactness to make comparisons to equivalent developmental stages of modern animals easy to perform. This method overlooks autapomorphic features of the fossils, but can avoid the overinterpretation that is sometimes caused by metamorphoses.
Embryonic fossils assignable to early developmental stages of Punctatus. A Collapsing cleavage stage Sn , possibly same stage as specimen in Fig. B Blastula or early gastrula developing spines within the egg membrane Sn C Later gastrula that has started mouth formation, showing five sectors divided by radial grooves and small sectors between grooves Sn D Split half portion of a possible gastrula with blastopore, inner cell mass, and spacious blastocoel Sn Micro-CT sections showing small gut.
A Young juvenile with a short blind gut and empty blastocoel Sn B Irregular trabeculae connecting the column and short gut Sn Trabeculae are thought to be metamorphic structures that appeared during fossilization. Micro-CT 3D-reconstruction of cleaving embryo.
Note the wide blastocoel and the size gradient of blastomeres, suggesting an animal-vegetal axis. Micro-CT 3D-reconstruction of young Punctatus emeiensis with four annular fringes. Note the very small gut suspended from the mouth and the nearly empty and spacious body cavity. We thank J. Han of Northwest University for his kind acceptance of the fossil observation and analyses; H.
Gong of Northwest University for his assistance in scanning electron microscopy; G. Suwa and F. Yoshitani of the University of Tokyo, and R. Otsuka of Hiroshima University for his constructive comments. We also thank two anonymous reviewers and the subject editor for their helpful comments of an earlier version of this manuscript.
Browse Subject Areas? Click through the PLOS taxonomy to find articles in your field. Abstract Background Microfossils of the genus Punctatus include developmental stages such as blastula, gastrula, and hatchlings, and represent the most complete developmental sequence of animals available from the earliest Cambrian.
Results Punctatus developed from a rather large egg, gastrulated in a mode of invagination from a coeloblastura, and then formed a mouth directly from the blastopore. Conclusions Contrary to proposed cnidarian affinity, the Punctatus body plan has basic differences from that of cnidarians, especially concerning a spacious body cavity separating ectoderm from endoderm.
Funding: The authors have no support or funding to report. Introduction Microfossils from the earliest sediments of the Cambrian, in the Kuanchuanpu Formation, China, collectively called Small Shelly Fossils SSFs , have provided exceptional fossils comprising reliable developmental series for some species [1] , [2] , [3] , [4]. Results Re-reconstruction of Punctatus Development Development of Punctatus was reconstructed starting from the hatchling stage, as this stage displays characters linking the embryonic and post-hatching stages Figure 1.
Download: PPT. Figure 1. Early development of Punctatus deduced from hatchling features. Oral Formation and Growth Pattern of Punctatus The mouth directly differentiated from the blastopore. Conical Remain is not a Thecal Tube, but Body Wall Stellate surfaces first appeared as early as the blastula or early gastrula stage within the egg membrane.
Discussion The SSF family Hexangulaconulariidae, which coexisted with Punctatus , is a key taxon for classifying Punctatus as a cnidarian because it has been proposed to be a morphological intermediate between conulariids and Punctatus [2].
Figure 4. Punctatus developmental pattern based on molecular developmental data. Conclusions Among the huge variety of cnidarians, only a small number of representatives have been fully studied. Figure 5. Comparison of developmental patterns among non-bilaterian phyla and Punctatus. Materials and Methods All fossil specimens were collected from Kua — [3] in the Kuanchuanpu Formation of the Shizhonggou Section at Ningqiang, Shaanxi, China between and Supporting Information.
Figure S1. Figure S2. Movie S1. Movie S2. Acknowledgments We thank J. References 1. J Paleontol — View Article Google Scholar 2. Science — View Article Google Scholar 3. Geobios — View Article Google Scholar 4. Chn Sci Bull — These pouches eventually fuse and expand to fill the space between the gut and the body wall, giving rise to the coelom. Another difference in organization of protostome and deuterostome embryos is expressed during cleavage.
Protostomes undergo spiral cleavage, meaning that the cells of one pole of the embryo are rotated, and thus misaligned, with respect to the cells of the opposite pole.
This is due to the oblique angle of cleavage relative to the two poles of the embryo. Deuterostomes undergo radial cleavage, where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the parallel up-and-down alignment of the cells between the two poles.
Figure 6. Protostomes and deuterostomes. Eucoelomates can be divided into two groups based on their early embryonic development. In protostomes, the mouth forms at or near the site of the blastopore and the body cavity forms by splitting the mesodermal mass during the process of schizocoely.
In deuterostomes, the mouth forms at a site opposite the blastopore end of the embryo and the mesoderm pinches off to form the coelom during the process of enterocoely. A second distinction between the types of cleavage in protostomes and deuterostomes relates to the fate of the resultant blastomeres cells produced by cleavage.
In addition to spiral cleavage, protostomes also undergo determinate cleavage. This means that even at this early stage, the developmental fate of each embryonic cell is already determined. A given cell does not have the ability to develop into any cell type other than its original destination.
Removal of a blastomere from an embryo with determinate cleavage can result in missing structures, and embryos that fail to develop. In contrast, deuterostomes undergo indeterminate cleavage, in which cells are not yet fully committed at this early stage to develop into specific cell types. Removal of individual blastomeres from these embryos does not result in the loss of embryonic structures.
In fact, twins clones can be produced as a result from blastomeres that have been separated from the original mass of blastomere cells. Unlike protostomes, however, if some blastomeres are damaged during embryogenesis, adjacent cells are able to compensate for the missing cells, and the embryo is not damaged. These cells are referred to as undetermined cells. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells , which have the ability to develop into any cell type until their fate is programmed at a later developmental stage.
One structure that is used in classification of animals is the body cavity or coelom. The body cavity develops within the mesoderm, so only triploblastic animals can have body cavities. Therefore body cavities are found only within the Bilateria. In other animal clades, the gut is either close to the body wall or separated from it by a jelly-like material. The body cavity is important for two reasons. Fluid within the body cavity protects the organs from shock and compression. In addition, since in triploblastic embryos, most muscle, connective tissue, and blood vessels develop from mesoderm, these tissues developing within the lining of the body cavity can reinforce the gut and body wall, aid in motility, and efficiently circulate nutrients.
To recap what we have discussed above, animals that do not have a coelom are called acoelomates. The major acoelomate group in the Bilateria is the flatworms, including both free-living and parasitic forms such as tapeworms.
In these animals, mesenchyme fills the space between the gut and the body wall. Although two layers of muscle are found just under the epidermis, there is no muscle or other mesodermal tissue around the gut. Flatworms rely on passive diffusion for nutrient transport across their body.
In pseudocoelomates , there is a body cavity between the gut and the body wall, but only the body wall has mesodermal tissue. In these animals, the mesoderm forms, but does not develop cavities within it. Major pseudocoelomate phyla are the rotifers and nematodes. Animals that have a true coelom are called eucoelomates ; all vertebrates, as well as molluscs, annelids, arthropods, and echinoderms, are eucoelomates.
The coelom develops within the mesoderm during embryogenesis. Of the major bilaterian phyla, the molluscs, annelids, and arthropods are schizocoels , in which the mesoderm splits to form the body cavity, while the echinoderms and chordates are enterocoels , in which the mesoderm forms as two or more buds off of the gut. These buds separate from the gut and coalesce to form the body cavity. In the vertebrates, mammals have a subdivided body cavity, with the thoracic cavity separated from the abdominal cavity.
The pseudocoelomates may have had eucoelomate ancestors and may have lost their ability to form a complete coelom through genetic mutations. Thus, this step in early embryogenesis—the formation of the coelom—has had a large evolutionary impact on the various species of the animal kingdom.
Organisms in the animal kingdom are classified based on their body morphology, their developmental pathways, and their genetic affinities. The relationships between the Eumetazoa and more basal clades Ctenophora, Porifera, and Placozoa are still being debated.
Generally, the simpler and often nonmotile animals display radial symmetry, which allows them to explore their environment in all directions. Animals with radial symmetry are also generally characterized by the development of two embryological germ layers, the endoderm and ectoderm, whereas animals with bilateral symmetry are generally characterized by the development of a third embryologic germ layer, the mesoderm.
Animals with three germ layers, called triploblasts, are further characterized by the presence or absence of an internal body cavity called a coelom.
The presence of a coelom affords many advantages, and animals with a coelom may be termed true coelomates or pseudocoelomates, depending the extent to which mesoderm lines the body cavity.
Coelomates are further divided into one of two groups called protostomes and deuterostomes, based on a number of developmental characteristics, including differences in zygote cleavage, the method of coelom formation, and the rigidity of the developmental fate of blastomeres. Figure Which of the following statements is false? Figure Which of the following statements about diploblasts and triploblasts is false? Using the following terms, explain what classifications and groups humans fall into, from the most general to the most specific: symmetry, germ layers, coelom, cleavage, embryological development.
Humans have body plans that are bilaterally symmetrical and are characterized by the development of three germ layers, making them triploblasts.
Humans have true coeloms and are thus eucoelomates. As deuterostomes, humans are characterized by radial and indeterminate cleavage. Explain some of the advantages brought about through the evolution of bilateral symmetry and coelom formation.
The evolution of bilateral symmetry led to designated head and tail body regions, and promoted more efficient mobility for animals. This improved mobility allowed for more skillful seeking of resources and prey escaping from predators. The appearance of the coelom in coelomates provides many internal organs with shock absorption, making them less prone to physical damage from bodily assault. A coelom also gives the body greater flexibility, which promotes more efficient movement.
The relatively loose placement of organs within the coelom allows them to develop and grow with some spatial freedom, which promoted the evolution of optimal organ arrangement.
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