Body plan

A body plan, Bauplan (German plural Baupläne), or ground plan is a set of morphological features common to many members of a phylum of animals.[1] The vertebrate body plan is one of many: invertebrates consist of many phyla.

This term, usually applied to animals, envisages a "blueprint" encompassing aspects such as symmetry, segmentation and limb disposition. Evolutionary developmental biology seeks to explain the origins of diverse body plans.

Body plans have historically been considered to have evolved in a flash in the Cambrian explosion, but a more nuanced understanding of animal evolution suggests gradual development of body plans throughout the early Palaeozoic.

Animal diversity
Modern groups of animals can be grouped by the arrangement of their body structures, so are said to possess different body plans.


The history of the discovery of body plans can be seen as a movement from a worldview centred on the vertebrates, to seeing the vertebrates (or chordates) as one phylum's body plan among many. Among the pioneering zoologists, Linnaeus identified two body plans outside the vertebrates; Cuvier identified three; and Haeckel had four, as well as the Protista with eight more, for a total of twelve. For comparison, the number of phyla recognised by modern zoologists has risen to 36.[1]

Linnaeus, 1735

In his 1735 book, Systema Naturæ, the Swedish botanist Linnaeus grouped the animals into quadrupeds, birds, "amphibians" (including tortoises, lizards and snakes), fish, "insects" (Insecta, in which he included arachnids, crustaceans and centipedes) and "worms" (Vermes). Linnaeus's Vermes included effectively all other groups of animals, not only tapeworms, earthworms and leeches but molluscs, sea urchins and starfish, jellyfish, squid and cuttlefish.[2]

Cuvier, 1817

Haeckel arbol bn
Haeckel's 'Monophyletischer Stambaum der Organismen' from Generelle Morphologie der Organismen (1866) with the three branches Plantae, Protista, Animalia

In his 1817 work, Le Règne Animal, the French zoologist Georges Cuvier combined evidence from comparative anatomy and palaeontology[3] to divide the animal kingdom into four body plans. Taking the central nervous system as the main organ system which controlled all the others, such as the circulatory and digestive systems, Cuvier distinguished four body plans or embranchements:[4]

  1. with a brain and a spinal cord (surrounded by skeletal elements)[4]
  2. with organs linked by nerve fibres[4]
  3. with two longitudinal, ventral nerve cords linked by a band with two ganglia below the oesophagus[4]
  4. with a diffuse nervous system, not clearly discernible[4]

Grouping animals with these body plans resulted in four branches: vertebrates, molluscs, articulata (including insects and annelids) and zoophytes or radiata.

Haeckel, 1866

Ernst Haeckel, in his 1866 Generelle Morphologie der Organismen, asserted that all living things were monophyletic (had a single evolutionary origin), being divided into plants, protista, and animals. His protista were divided into moneres, protoplasts, flagellates, diatoms, myxomycetes, myxocystodes, rhizopods, and sponges. His animals were divided into groups with distinct body plans: he named these phyla. Haeckel's animal phyla were coelenterates, echinoderms, and (following Cuvier) articulates, molluscs, and vertebrates.[5]

Gould, 1979

Stephen J. Gould explored the idea that the different phyla could be perceived in terms of a Bauplan, illustrating their fixity. However, he later abandoned this idea in favor of punctuated equilibrium.[6]


20 out of the 36 body plans originated in the Cambrian period,[7] in the "Cambrian explosion",[8] However, complete body plans of many phyla emerged much later, in the Palaeozoic or beyond.[9]

The current range of body plans is far from exhaustive of the possible patterns for life: the Precambrian Ediacaran biota includes body plans that differ from any found in currently living organisms, even though the overall arrangement of unrelated modern taxa is quite similar.[10] Thus the Cambrian explosion appears to have more or less completely replaced the earlier range of body plans.[7]

Genetic basis

Genes, embryos and development together determine the form of an adult organism's body, through the complex switching processes involved in morphogenesis.

Developmental biologists seek to understand how genes control the development of structural features through a cascade of processes in which key genes produce morphogens, chemicals that diffuse through the body to produce a gradient that acts as a position indicator for cells, turning on other genes, some of which in turn produce other morphogens. A key discovery was the existence of groups of homeobox genes, which function as switches responsible for laying down the basic body plan in animals. The homeobox genes are remarkably conserved between species as diverse as the fruit fly and humans, the basic segmented pattern of the worm or fruit fly being the origin of the segmented spine in humans. The field of animal evolutionary developmental biology ('Evo Devo'), which studies the genetics of morphology in detail, is rapidly expanding[11] with many of the developmental genetic cascades, particularly in the fruit fly Drosophila, catalogued in considerable detail.[12]

See also


  1. ^ a b Valentine, James W. (2004). On the Origin of Phyla. p. 33. ISBN 978-0226845487.
  2. ^ Linnaeus, Carolus (1735). Systema naturae, sive regna tria naturae systematice proposita per classes, ordines, genera, & species. Leiden: Haak. pp. 1–12.
  3. ^ Reiss, John (2009). Not by Design: Retiring Darwin's Watchmaker. University of California Press. p. 108. ISBN 978-0-520-94440-4.
  4. ^ a b c d e De Wit, Hendrik Cornelius Dirk De Wit. Histoire du Développement de la Biologie, Volume III, Presses Polytechniques et Universitaires Romandes, Lausanne, 1994, p. 94-96. ISBN 2-88074-264-1
  5. ^ Haeckel, Ernst. Generelle Morphologie der Organismen : allgemeine Grundzüge der organischen Formen-Wissenschaft, mechanisch begründet durch die von Charles Darwin reformirte Descendenz-Theorie. (1866) Berlin
  6. ^ Bowler, Peter J. (2009). Evolution: the History of an Idea. California, p. 364.
  7. ^ a b Erwin, Douglas; Valentine, James; Jablonski, David (1997). "Recent fossil finds and new insights into animal development are providing fresh perspectives on the riddle of the explosion of animals during the Early Cambrian". American Scientist (March–April).
  8. ^ Erwin, D. H. (1999). "The Origin of Bodyplans". Integrative and Comparative Biology. 39 (3): 617–629. doi:10.1093/icb/39.3.617.
  9. ^ Budd, G. E.; Jensen, S. (2000). "A critical reappraisal of the fossil record of the bilaterian phyla". Biological Reviews of the Cambridge Philosophical Society. 75 (2): 253–95. doi:10.1111/j.1469-185X.1999.tb00046.x. PMID 10881389.
  10. ^ Antcliffe, J. B.; Brasier, M. D. (2007). "Charnia and sea pens are poles apart". Journal of the Geological Society. 164: 49–51. doi:10.1144/0016-76492006-080.
  11. ^ Hall, Brian K. (28 March 2005). "Evo Devo is the New Buzzword..." Retrieved 13 September 2014.
  12. ^ Arthur, Wallace. (1997). Animal Body Plans. Cambridge England: Cambridge University Press. ISBN 0-521-77928-6.

External links

Anatomical terms of location

Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans.

All vertebrates (including humans) have the same basic body plan – they are strictly bilaterally symmetrical in early embryonic stages and largely bilaterally symmetrical in adulthood. That is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate) organisms as well.

While these terms are standardized within specific fields of biology, they are unavoidable, sometimes dramatic differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories.


Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The kingdom Animalia includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

Most living animal species are in the Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes—in which many groups of invertebrates are found, such as nematodes, arthropods, and molluscs—and the deuterostomes, containing both the echinoderms as well as the chordates, the latter containing the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous with Animalia) and the Protozoa, single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the evolutionary relationships between animal taxa.

Humans make use of many other animal species for food, including meat, milk, and eggs; for materials, such as leather and wool; as pets; and as working animals for power and transport. Dogs have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Non-human animals have appeared in art from the earliest times and are featured in mythology and religion.


The anus (from Latin anus meaning "ring", "circle") is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may include: matter which the animal cannot digest, such as bones; food material after all the nutrients have been extracted, for example cellulose or lignin; ingested matter which would be toxic if it remained in the digestive tract; and dead or excess gut bacteria and other endosymbionts.

Amphibians, reptiles, and birds use the same orifice (known as the cloaca) for excreting liquid and solid wastes, for copulation and egg-laying. Monotreme mammals also have a cloaca, which is thought to be a feature inherited from the earliest amniotes via the therapsids. Marsupials have a single orifice for excreting both solids and liquids and, in females, a separate vagina for reproduction. Female placental mammals have completely separate orifices for defecation, urination, and reproduction; males have one opening for defecation and another for both urination and reproduction, although the channels flowing to that orifice are almost completely separate.

The development of the anus was an important stage in the evolution of multicellular animals. It appears to have happened at least twice, following different paths in protostomes and deuterostomes. This accompanied or facilitated other important evolutionary developments: the bilaterian body plan, the coelom, and metamerism, in which the body was built of repeated "modules" which could later specialize, such as the heads of most arthropods, which are composed of fused, specialized segments.


The bilateria [ˌbɪlaˈtɛrɪ.a], bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.The bilateria are a major group of animals, including the majority of phyla but not sponges, ctenophores, placozoans, and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).

Fish locomotion

Fish locomotion is the variety of types of animal locomotion used by fish, principally by swimming. This however is achieved in different groups of fish by a variety of mechanisms of propulsion in water, most often by wavelike movements of the fish's body and tail, and in various specialised fish by movements of the fins. The major forms of locomotion in fish are anguilliform, in which a wave passes evenly along a long slender body; sub-carangiform, in which the wave increases quickly in amplitude towards the tail; carangiform, in which the wave is concentrated near the tail, which oscillates rapidly; thunniform, rapid swimming with a large powerful crescent-shaped tail; and ostraciiform, with almost no oscillation except of the tail fin. More specialised fish include movement by pectoral fins with a mainly stiff body, as in the sunfish; and movement by propagating a wave along the long fins with a motionless body in fish with electric organs such as the knifefish.

In addition, some fish can variously "walk", i.e., move over land, burrow in mud, and glide through the air.


A humanoid (; from English human and -oid "resembling") is something that has an appearance resembling a human without actually being one. The earliest recorded use of the term, in 1870, referred to indigenous peoples in areas colonized by Europeans. By the 20th century, the term came to describe fossils which were morphologically similar, but not identical, to those of the human skeleton.Although this usage was common in the sciences for much of the 20th century, it is now considered rare. More generally, the term can refer to anything with distinctly human characteristics or adaptations, such as possessing opposable anterior forelimb-appendages (i.e. thumbs), visible spectrum-binocular vision (i.e. having two eyes), or biomechanic plantigrade-bipedalism (i.e. the ability to walk on heels and metatarsals in an upright position). Science fiction media frequently present sentient extraterrestrial lifeforms as humanoid as a byproduct of convergent evolution theory.


In developmental biology, invagination is a mechanism that takes place during gastrulation. This mechanism or cell movement happens mostly in the vegetal pole. Invagination consists of the folding of an area of the exterior sheet of cells towards the inside of the blastula. In each organism, the complexity will be different depending on the number of cells. Invagination can be referenced as one of the steps of the establishment of the body plan. The term, originally used in embryology, has been adopted in other disciplines as well.

There is more than one type of movement for invagination. Two common types are axial and orthogonal. The difference between the production of the tube formed in the cytoskeleton and extracellular matrix. Axial can be formed at a single point along the axis of a surface. Orthogonal is linear and trough.

Inversion (evolutionary biology)

In evolutionary developmental biology, inversion refers to the hypothesis that during the course of animal evolution, the structures along the dorsoventral (DV) axis have taken on an orientation opposite that of the ancestral form.

Inversion was first noted in 1822 by the French zoologist Étienne Geoffroy Saint-Hilaire, when he dissected a crayfish (an arthropod) and compared it with the vertebrate body plan. The idea was heavily criticised, but periodically resurfaced, and is now supported by some molecular embryologists.


The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term jaws is also broadly applied to the whole of the structures constituting the vault of the mouth and serving to open and close it and is part of the body plan of most animals.


Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen.


Mites are small arthropods belonging to the class Arachnida and the subclass Acari (also known as Acarina). The term "mite" refers to the members of several groups in Acari but it is not a clade as it spans two different groups of arachnids; it also excludes the ticks, order Ixodida. Mites and ticks are characterised by the body being divided into two regions, the cephalothorax or prosoma (there is no separate head), and an opisthosoma. The scientific discipline devoted to the study of ticks and mites is called acarology.

Most mites are tiny, less than 1 mm (0.04 in) in length, and have a simple, unsegmented body plan. Their small size makes them easily overlooked; some species live in water, many live in soil as decomposers, others live on plants, sometimes creating galls, while others again are predators or parasites. This last group includes the commercially important Varroa parasite of honey bees, as well as the scabies mite of humans. Most species are harmless to humans but a few are associated with allergies or may transmit diseases.

Mole (animal)

Moles are small mammals adapted to a subterranean lifestyle (i.e., fossorial). They have cylindrical bodies; velvety fur; very small, inconspicuous ears and eyes; reduced hindlimbs; and short, powerful forelimbs with large paws adapted for digging.

The term mole is especially and most properly used for "true moles" of the Talpidae family in the order Eulipotyphla, which are found in most parts of North America, Asia, and Europe, although it may also refer to unrelated mammals of Australia and southern Africa that have convergently evolved the "mole" body plan. The term is not applied to all talpids; e.g., desmans and shrew-moles differ from the common definition of "mole".

Moles are known pests to human activities such as agriculture, lawncare, and gardening. However, they do not eat plant roots; they only cause damage indirectly as they eat earthworms and other small invertebrates in the soil.


Najash is an extinct basal snake from the Late Cretaceous Candeleros Formation of Patagonia. Like a number of other Cretaceous and living snakes it retained hindlimbs, but Najash is unusual in having well-developed legs that extend outside the rib cage, and a pelvis connected to the spine. Fossils of Najash were found in the terrestrial Candeleros Formation, in Rio Negro Province, Argentina, and date to roughly 90 million years ago. The skull and spine of Najash show primitive features that resemble other Cretaceous snakes, such as Dinilysia and Madtsoiidae. Also, several characteristicts of the neck and tail of Najash and Dinilysia show how the body plan of snakes evolved from a lizard-like ancestor.Najash had not lost its sacrum, the pelvic bone composed of several fused vertebrae, nor its pelvic girdle which are absent in modern snakes, and in all other known fossil snakes as well. Nearly all phylogenetic analysis place Najash as an early shoot of the snake tree, outside of all living snakes.The generic name comes from the biblical legged snake of Genesis, Nahash, who tempted Adam and Eve to eat from a forbidden fruit tree.


In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

Segmentation (biology)

Segmentation in biology is the division of some animal and plant body plans into a series of repetitive segments. This article focuses on the segmentation of animal body plans, specifically using the examples of the taxa Arthropoda, Chordata, and Annelida. These three groups form segments by using a "growth zone" to direct and define the segments. While all three have a generally segmented body plan and use a growth zone, they use different mechanisms for generating this patterning. Even within these groups, different organisms have different mechanisms for segmenting the body. Segmentation of the body plan is important for allowing free movement and development of certain body parts. It also allows for regeneration in specific individuals.


Somites (outdated: primitive segments) are divisions of the body of an animal or embryo. The divisions are also known as metameric segments.

Somites are bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide into the sclerotomes, myotomes, syndetomes, and dermatomes that give rise to the vertebrae of the vertebral column, rib cage, and part of the occipital bone; skeletal muscle, cartilage, tendons, and skin (of the back).The word somite is also used in place of the word metamere. In this definition, the somite is a homologously paired structure in an animal body plan, such as is visible in annelids and arthropods.

Tradeoffs for locomotion in air and water

Certain species of fish and birds are able to locomote in both air and water, two fluid media with very different properties. A fluid is a particular phase of matter that deforms under shear stresses and includes any type of liquid or gas. Because fluids are easily deformable and move in response to applied forces, efficiently locomoting in a fluid medium presents unique challenges. Specific morphological characteristics are therefore required in animal species that primarily depend on fluidic locomotion. Because the properties of air and water are so different, swimming and flying have very disparate morphological requirements. As a result, despite the large diversity of animals that are capable of flight or swimming, only a limited number of these species have mastered the ability to both fly and swim. These species demonstrate distinct morphological and behavioral tradeoffs associated with transitioning from air to water and water to air.

Tube worm

A tube worm is any worm-like sessile invertebrate that anchors its tail to an underwater surface and secretes around its body a mineral tube, into which it can withdraw its entire body.

Tube worms are found among the following taxa:

Annelida, the phylum containing segmented worms

Polychaetea, the class containing bristle worms

Canalipalpata, the order containing bristle-footed annelids or fan-head worms

Siboglinidae, the family of beard worms

Riftia pachyptila, a species known as giant tube worms

Lamellibrachia, a genus

Serpulidae, a family

Sabellidae, the family containing feather duster worms

Phoronida, the phylum containing horseshoe worms

Microconchida, an order of extinct tubeworms

Kuphus polythalamia, a bivalve mollusc species whose common name is giant tube worm

Other vertebrates
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Evolution of genetic systems
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Influential figures
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