What is body plan symmetry?

views updated May 29 2018

The term "body plan" refers to the general similarities in development and form and function among members of a particular phylum. Another name for these similarities is baüplan, which is the German word for "body plan."

A body plan is a group of structural and developmental characteristics that can be used to identify a group of animals, such as a phylum. All members of a particular group share the same body plan at some point during their development—in the embryonic, larval, or adult stage. Biologists have long observed that anatomy and embryology reflect shared underlying structural plans. These plans can be used to define taxonomic groups (usually phyla) and to construct hierarchical classifications within groups (organisms with similar body plans tend to be more closely related).

Similarities and differences in adult shape and form, as well as the developmental pattern of embryos, provide the framework for modern taxonomic classification. These comparisons are the basis of phylogenetic systematics. Embryonic development is relatively consistent among animals with similar body plans, although similar larval forms may give rise to very different adults in some groups. The timing, pattern, and scale of developmental events determine the shape of an organism, and closely related groups are more likely to share structural and developmental similarities than those that are more distantly related. Homologous structures and developmental stages—those that are similar among related groups because they are inherited from a common ancestor—are the basis of modern biological classification.

The fossil record suggests that metazoans (organisms with multiple cell and tissue types) first appeared about 500,000 years ago early in the Cambrian period. It is likely, however, that soft-bodied forms were present well before this but left no fossilized remains. Metazoans rapidly diversified into myriad forms that eventually gave rise to the diversity of metazoans we have today. Biologists refer to this historic event in the history of animal life as the Cambrian Radiation, sometimes referred to as the Cambrian Explosion. All living animals are descendents of a common ancestor that existed at the beginning of the Cambrian period, and their various evolutionary paths were established by the end of this biological supernova. A few body plans did not survive into the present, but the majority of readily fossilized metazoan body plans can still be found today. The distribution of diverse body plans among living metazoans provides a record of the evolutionary history of this group that dates back to its origin.

Multicellularity in organisms permits specialization of cell structure and function. During the Cambrian Radiation, an increased overall complexity and the subsequent differentiation of embryonic and adult cells and tissues, a widespread phenomenon among metazoans called compartmentalization, provided the opportunity for evolutionary experimentation and innovation.

New combinations of cells and tissues led to greater complexity and the exploitation of new ecological resources.

Important differences among body plans are present in the embryo although they may be apparent at any stage during the development of a given group. Conditions presented early in development set in motion a cascade of changes in cell growth, proliferation, and differentiation that operate throughout development to produce the body plans specific to a particular group of organisms.

Body plans vary among phyla in terms of egg-cleavage patterns (how the egg divides in early development), gastrulation , axis specification, and embryonic cell structure. The egg may be completely divided by the cleavage furrow (holoblastic cleavage), or only a portion of the cytoplasm may be cleaved (meriblastic cleavage) as in bird eggs. Deuterostomes , such as echinoderms and chordates, develop by radial cleavage. In this form of cleavage, the daughter cells sit on top of previous cells. Protostomes , such as mollusks, annelids, and arthropods, develop by spiral cleavage (the daughter blastomeres are not direclty over or beside each other but are tilted to the left or right 45 degrees).

Gastrulation is the coordinated movement of cells and tissue in the embryo that determines later cell and tissue interactions. Gastrulation involves the combination of cell and tissue. These combination types differ among phyla.

Axis formation in the embryo is responsible for determining patterns of symmetry and polarity. Organisms may be asymmetrical (no symmetry) or symmetrical (a single line, or plane, of symmetry). Symmetry may be spherical, radial, or bilateral. Animals with spherical symmetry, like sea urchins, have a hollow globe of cell layers organized around a central point. Animals with radial symmetry, like jellyfishes, have body parts that radiate from a central point, like the spokes of a wheel. Animals with bilateral symmetry , like earthworms, have bodies that if cut lengthwise, form right and left halves that are mirror images.

Bilateral symmetry is a critical prerequisite for the concentration of sensory organs and the development of the head. Dorsal -ventral (back-belly), anterior-posterior (mouth-anus), and right-left axes are specified in different ways among phyla. The primary body axes of annelids and vertebrates, for example, are determined by different mechanisms during early development. In most cases, the presence of multiple embryonic developmental axes is associated with cell diversity and tissue complexity. Cells and tissues in the embryo give rise to all classes of cells, tissues, and structures present in the adult stage. The specific fate of embryonic cells and tissues is determined early in development and varies among body plans.

Most metazoan body plans can be described as a "tube-within-a-tube," with a body wall made up of layers of different tissue types surrounding a central cavity. In almost all metazoans, the body wall has three cell layers (ectoderm, mesoderm, and endoderm), although some, such as sponges (Porifera), have no organized cell layers, and others, such as jellyfishes (Cnidaria ) have only two layers in the adult. Multicellular metazoan ancestors had an inside-outside, two-layered organization with an endoderm and ectoderm. In triploblasts , such as flatworms, a middle layer of mesoderm also evolved.

The body wall surrounds a coelum (central cavity) between the digestive tract and body wall that is completely lined by mesoderm. The coelom allows the digestive system and body wall to move independently. Because of this, internal organs can be more complex. The coelom may also serve as a storage area for eggs and sperm, facilitating development of these gametes within the animal body. Coelomic fluid helps in respiration and circulation by diffusing nutrients and in excretion by accumulating wastes. This fluid has the same function as several organ systems in the higher animals. In addition, coelomic fluid protects internal organs and serves as a hydrostatic skeleton. Metazoans have a mouth at one end of the coelom and an anus at the other. Protostomes develop so that the first opening in the embryo is the mouth (the word "protostome" means "first mouth"). Deuterostomes develop an anus first, then a mouth.

Of the thirty-five living phyla of metazoans, the ten largest contain nearly 2.5 million species in total, the other twenty-five account for only 5,000. Although some phyla have obviously been more successful than others in terms of the sheer number of species that they contain, all existing metazoan body plans are survivors of the Cambrian Radiation. The following is a description of the body plans of the ten most diverse metazoan phyla, presented in order of increasing complexity.

Porifera: Sponges

Sponges have a diploblastic embryo, which means they have a two-layered body wall (ectoderm and endoderm but no mesoderm). Adults are sessile (nonmobile and usually fixed to a single point) and have no coelom. Sponges have flagellated cells that move water around the body, and an internal skeleton with spicules, needle-shaped skeletal elements that occur in the matrix between the epidermal and collar cells. Adult sponges have no definite nervous system.

Cnidaria: Corals, Jellyfishes, and Anemones

Cnidarians have a body that is a simple, soft-walled sac. Cnidarians have two distinctive body forms, a mobile, bell-shaped medusa (jellyfish, for example) or a sessile polyp (sea anemones, for example). Either or both forms may be present during development, depending on the species. Cnidarians may live on their own, as do anemones, or live in colonies, as do corals and jellyfishes.

All cnidarians have radial symmetry. They are diploblastic , which means they have two embryonic tissue layers, the ectoderm and endoderm, which give rise to the ectodermis and gastrodermis of the adult. The latter layers enclose a single opening, the enteron, or "inner cavity."

They have a mouth but no anus; and have a central body cavity called a coelenteron (hollow gut), and a nerve net, which serves as a primitive nervous system. Cnidarians are the only metazoans that have tentacles with nematocysts (stinging cells) and statocysts (organs that sense orientation).

Platyhelminthes: Flatworms

Flatworms are bilaterally symmetrical and have flattened, wormlike bodies. All platyhelminthes are triploblastic. They have unique flagellated cells called flame cells, which regulate the contents of extracellular fluid and are used for excretion, and a nervous system with a simple brain.

Rotifera: Wheeled Animals

Rotifers have several complex traits that are further developed in other phyla. The rotifer body is unsegmented, bilaterally symmetrical, and spherical with a bifurcate (split) foot and anterior wheel organ and a cuticle (extracellular protective layer). Rotifers feed using a pharynx with jaws. They have protonephridia, a primitive excretory organ, and a simple nervous system with vision receptors.

Nematoda: Roundworms

Nematodes have triploblastic embryos and cylindrical, unsegmented bodies in the adult stage. They have a pseudocoelom , a closed, fluid-containing cavity that acts as a hydrostatic skeleton to maintain body shape, circulate nutrients, and hold the major body organs. Nematodes also have a cuticle without cilia, longitudinal muscle fibers, a triradiate (three-chambered) pharynx, and an excretory system that consists of gland cells and canals.

Molluska: Slugs, Snails, and Clams

The mollusk body has a head and a foot, and a mantle , a membranous or muscular structure that surrounds the visceral mass (internal organs) and secretes a shell if one is present (as in clams and branchiopods). Mollusks have an alimentary canal, a relatively complex nervous system, respiratory gills, and an active circulatory system with blood and a hemocoele, an enlarged, blood-filled space. Some groups of mollusks have a reduced coelom.

Annelida: Segmented Worms

The annelid body is bilaterally symmetrical, segmented, and fluid filled. Annelids have a hydrostatic skeleton, which supports the body through the pressure of fluid contained within body cavities. The external surface of the body is protected by a cuticle. Annelids also have chaetae, or bristles, and a triploblastic body wall. The annelid nervous system consists of paired nerves and ganglia (clusters of neuron bodies or soma) arranged along the length of the body. They have simple excretory organs called nephridia, or coelomoducts, and a closed, tubular circulatory system.

Arthropoda: Crustaceans, Spiders, and Insects

Arthropods have triploblastic embryos. Their bodies are bilaterally symmetrical, with metameric segmentation, in which each repeating segment is similar to the next. Arthropods have an exoskeleton, a hard, jointed, external covering that encloses the muscles and organs, made of chitin (a tough, flexible carbohydrate); paired, jointed appendages; and one or more pairs of jaws. The digestive system consists of a tubular gut. Arthropods have striated muscles , a ventral nerve cord of segmental ganglia, ciliated sense organs, a reduced coelom, a haemocoel, and a heart that pumps a circulatory fluid called haemolymph.

Echinodermata: Sea Urchins, Starfishes, and Sea Cucumbers

Echinoderms have a swimming larval stage called a pluteus and a non-swimming, headless adult stage with pentamerous (five-sided) symmetry. Echinoderms have a coelom that is divided into three sections, and an internal mesodermal skeleton (a supportive framework of connective tissue ) with calcium carbonate spicules (conical masses of hard, shell-like matrial). Echinoderms have digestive systems but lack excretory organs. They have a water vascular system (also called the ambulacral system), which is a set of hydraulic canals derived from the coelom and equipped with tube feet, and which is used for gas exchange, movement, food handling, and sensory reception.

Chordata: Chordates, Including Vertebrates

Chordate embryos (and adults) are triploblastic. Chordate larvae and adults are bilaterally symmetrical and have a well-defined anterior-posterior axis (the "head" is easily identified from the "tail"). Adults have a complex nervous system with a dorsal nerve chord and notochord (some groups, including vertebrates, have a brain), various sense organs, gill slits, and a well-developed digestive tract. Chordates reproduce sexually.

see also Allometry.

Andrew G. Gluesenkamp

Bibliography

Gilbert, Scott F. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, Inc., 1997.

Gilbert, Scott F., and Ann M. Raunio, eds. Embryology: Constructing the Organism. Sunderland, MA: Sinauer Associates, Inc., 1997.

Kalthoff, Klause. Analysis of Biological Development, 2nd ed. Boston: McGraw-Hill, Inc., 2001.

Raff, Rudolph A. The Shape of Life. Chicago: University of Chicago Press, 1996.

Raff, Rudolph A., and Thomas C. Kaufman. Embryos, Genes, and Evolution: The Developmental-Genetic Basis of Evolutionary Change. New York: Macmillan, 1983.

views updated May 18 2018

body plan The ‘blueprint’ according to which an organism develops a predetermined number, arrangement, and size of body components. The body plan is embodied and implemented by the organism's genes (e.g. homeotic genes), but can be influenced by environmental factors, such as malnutrition or exposusre to toxic agents (e.g. teratogens).

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