Symmetry in general sense can simply be defined as an arrangement of parts of a body or a shape into geometrical designs such that the body is divisible into proportionate and balanced similar parts in those halves arranged along lines, planes or axes.
Biological symmetry, therefore, refers to the symmetry observed in the arrangement of various parts of the body in living organisms, including plants, animals, fungi, and bacteria. The external symmetry can be observed easily by just looking at an organism, however, internal symmetry may require special laboratory procedures to observe.
Symmetry in animals was developed as a concept by Ernst Haeckel. It can be of different types. Some of the major types of symmetries found in animals are discussed below, as follows:
Asymmetry
Animals are said to be asymmetrical if they do not possess any symmetry. Asymmetry in animals is typically considered to be a primitive character, however, some species of animals have evolved asymmetry as an important adaptation. Many members of the phylum Porifera (sponges) and protozoans are asymmetrical, though some poriferans are radially symmetrical. Asymmetry can be observed in animals ranging from lower to higher invertebrates up to chordates. For e.g: Flatfish has both eyes on the same side of its head, Internal asymmetry in human-beings (liver on right side of body but no liver present towards left side, differences between the size and shape of lungs, positioning of kidneys, right or left-handedness in human-beings showing asymmetry in functional parts of the brain) etc.
Spherical Symmetry
An organism is said to be spherically symmetrical if it is divisible into two identical halves through any plane that runs through the center of the organism. True spherical symmetry is not found in animal body plans, therefore, animals show approximate spherical symmetry. This type of symmetry is found in animals whose bodies are spherical or ball-like in shape. Spherical symmetry is suitable for animals that are free-floating in fluid medium or for animals that do rolling movements or in sedentary habits where food is available in all directions. Body of such organisms bear locomotory organs like cilia or tentacles all around the body in a radiating manner for this purpose. Spherical symmetry is found in some protozoans like Volvox, Actinophrys (Heliozoa) and Thalassicola (Radiolaria).
Radial Symmetry
Organisms are said to possess radial symmetry if they show a repeating pattern around a central axis such that they can be separated into several identical parts when cut through the central plane or axis. Typically, this involves repeating a body part ‘n’ number of times, such as 4, 5, or 6 times around the axis – referred to as tetramerism, pentamerism and hexamerism respectively. Such organisms generally do not exhibit left or right sides but do have a dorsal and a ventral surface. Radially symmetric animals are generally symmetrical about an axis extending from the center of the oral surface , which contains the mouth (may be dorsal or ventral), to the center of the aboral end (generally opposite to the oral end). Animals classified into the phyla Porifera, Cnidaria and Echinodermata show radial symmetry, though many sea anemones and some corals within the phylum Cnidaria have bilateral symmetry (as perceived by a structure called as siphonoglyph).
Radial symmetry can be of different types depending upon the number of repetitive parts in which it can be divided symmetrically. For instance: triradial symmetry (3 radially symmetrical parts) is present in Trilobozoa (extinct sessile animals originally classified in phylum Cnidaria) from the Late Ediacaran period. Tetraradial symmetry (4 radially symmetrical parts) can be observed in jellyfish Aurelia marginalis. Echinoderms such as sea stars, sea urchins, and sea lilies are pentaradially symmetrical (5 radially symmetrical parts or multiple of 5) as adults (but their larvae are bilaterally symmetrical), with five arms arranged around the mouth. Hexamerism / Hexaradial symmetry (6 radially symmetrical parts or multiple of six) is found in the corals and sea anemones (class Anthozoa), which are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia have a hexameric body plan (their polyps have six-fold internal symmetry and a number of tentacles that is a multiple of six). Similarly, octamerism / octaradial symmetry (8 radially symmetrical parts or multiple of 8) is found in corals of the subclass Octocorallia (these have polyps with eight tentacles and octameric radial symmetry). It is also interesting to note that commonly known 8-armed octopus has bilateral symmetry, and not octaradial or radial symmetry.
French naturalist and zoologist George Cuvier classified animals with radial symmetry into the taxon Radiata (Zoophytes), but it is now generally accepted to be just a polyphyletic group with no common ancestor. Radial symmetry is especially suitable for sessile animals where food is planktonic and abundantly available in all directions. Various organs for capturing food are therefore arranged radially so that the animal does not have to move in search of food that is floating all around the body, such as, sea anemones (sessile), jellyfish (free-floating), and starfish (sluggish or slow-moving).
Biradial Symmetry
Biradial symmetry is also perhaps a variant of radial symmetry, however, it is quite distinctive from other types of radial symmetries. An animal is said to have biradial symmetry when its morphological features (internal or external) exhibit both bilateral as well as radial symmetry. Unlike radially symmetrical organisms which can be divided equally along many planes, biradially symmetrical organisms can be cut equally along two planes (four parts) only.
Biradial symmetry is the characteristic feature of ctenophores (Acnidaria, also called comb-jellies) which are not sedentary but floating animals. The two planes of symmetry in ctenophores are: (1) the plane of the tentacles and (2) the plane of the pharynx / comb-plates. Tentacles demonstrate bilateral symmetry whereas comb plates show radial symmetry and the animal takes advantage of both symmetries for food hunting and active swimming. Animals such as Pleurobrachia have oval body on which eight comb plates are radially arranged like bands and are used for swimming, whereas mouth, anal pore and statocysts are placed on the anterio-posterior axis. They also have a pair of retractile tentacles that bear colloblasts (lasso cells) which secrete sticky substance that helps in capturing planktonic food. Therefore, the organism can be divided into four parts about a central axis in which each opposing segment is identical to each-other while adjacent segments are dissimilar (biradial symmetry).
In addition to this minor phyla, evidences for biradial symmetry has also been found in the ‘perfectly radial’ freshwater polyp Hydra (a cnidarian). Biradial symmetry, especially when considering both internal and external features, is more common than originally accounted for. This type of symmetry could represent an intermediate stage in the evolution of bilateral symmetry from a radially symmetric ancestor.
Bilateral Symmetry
Animals are said to be bilaterally symmetrical when they contain a single plane of symmetry, the sagittal plane, which divides the organism into two (roughly) mirror images – left and right halves – approximate reflectional symmetry. Animals with bilateral symmetry are classified into a large group called the bilateria. The bilateria contains 99% of all known animals (comprising over 32 phyla and approximately 1 million described species). All the bilaterians have some asymmetrical features though; for example, the internal human organs such as heart, liver, lungs and kidneys are positioned asymmetrically despite the body having external bilateral symmetry.
The bilateral symmetry of bilaterians is a complex trait which develops due to the expression of several genes. The bilateria have two axes of polarity: 1) Antereoposterior Axis / Anterior-Posterior Axis (AV), and 2) Dorsoventral axis / Dorsal-Ventral Axis (DV). The AP axis can be visualised as an imaginary axis running from the head or mouth to the tail or other end of an organism, whereas, the DV axis runs perpendicular to the AP axis. During development, the AP axis is always specified before the DV axis which is known as the second embryonic axis. The AP axis in embryo is essential in defining the polarity of bilateral organisms, allowing the development of a front and back to give the organism a directional sense. The front end encounters the environment before the rest of the body so sensory organs such as brain, eyes, ears etc. tend to be clustered there. The same reason fits to explain why a mouth (or a proboscis and / or a tongue) develops at the front end; because it is the first part of the body to encounter food, thus, enhancing the chances of prey-detection, chemoreception, gustation, feeding and consequentially maximising the chances of survival. Therefore, a distinct head with sense organs connected to a central nervous system tends to develop in bilateria. This pattern of development (with a distinct head and tail) is called as cephalization. It is also remarkable to note that the development of an AP axis is particularly important in locomotion since bilateral symmetry gives the body a capability to move in a definitive direction as well as it allows body-streamlining to reduce frictional drag (exerted by air in terrestrial environment, or water in aquatic environment).
Platyhelminths are the first bilaterally symmetrical animals in group bilateria. Therefore, amongst the major phyla platyhelminths, nemathelminths, annelids, arthropods, (some) molluscs, echinoderms (larval stages) and all chordates are bilaterally symmetrical.