The Latin word symmetry He arrived in Spanish as symmetry , a term that refers to the correspondence between the size, position and shape of the components of a whole. Radial , meanwhile, is linked to radio (the segment that relates a circle to the center of the circle).
If we position ourselves in the field of biology , symmetry refers to the arrangement of the body and its parts with respect to an axis, a center or a plane. In this sense, we talk about radial symmetry when a heteropolar type shaft (that is, it is different at its ends).
Radial symmetry, therefore, divides the body in a oral side (the one that houses the mouth) and a aboral side , also know as abactinal side (the opposite side to the oral side). Said division is made by an axis on which the planes of symmetry that define, in turn, the per-radial positions are determined.
In comparison with the bilateral symmetry (which divides the body into a left half and a right half), radial symmetry is considered as primitive. It is important to mention that bilateral symmetry favors cephalization and the development of a central nervous system.
Between the animals which have radial symmetry of different kinds, it is possible to find echinoderms , anemones and sponges . One of the peculiarities of animals with some type of radial symmetry is that they usually have sensory receptors that are distributed regularly throughout the body's periphery.
In more general terms, we can say that there are large groups of organisms that usually present radial symmetry, as well as their specializations. Among them are sessile animals, which are characterized by the absence of an organ that can be used as a support or foot .
In the case of plants, sessile leaves are those that have no union with the petiole or stem; sessile flowers, meanwhile, do not have peduncles; anthers, on the other hand, are considered that way when their filament is very short or absent. The sessile animals are those aquatic organisms that develop rooted, seized or attached to their substratum .
This relationship of sessile animals and their substrates is not limited to the stage of growth, since they remain with them and do not move on them. It is common for organizations belonging to the community called bentos belong to this classification (bentos are those that live in the bottom of a ecosystem water).
Some of the organisms considered sessile, many of which have radial symmetry, are the following: corals, sponges, certain bivalve molluscs (among which are mussels and oysters, although clams are left out), brachiopods and The bryozoans. Cirripeda crustaceans are also sessile, and this can be seen in scales and barnacles.
As can be seen in the definition of sessile animals, one of the features mentioned is the sedentary lifestyle , and this also appears in several examples of organisms with radial symmetry.
On the other hand are the pelagic animals drifting . These are those organisms that live in the part of the ocean that is not above the continental shelf. This area is called pelagic (it is also possible to refer to it as "piélago") and represents the image that most people have of the ocean itself, since when it approaches the coast we usually talk about "sea".
Radial symmetry benefits pelagic animals drifting as it gives them the ability to move in any direction equally, thanks to the regular arrangement of their receivers and structures.