Symmetry

SYMMETRY, a morphological phenomenon consisting in the fact that similar parts of an organism's body or its organs are repeated around a certain axis or occupy the same position in relation to a certain plane. Symmetry is so widespread in the world of organisms that finding examples of its absence is extremely difficult; even such seemingly 'formless' organisms as amoebas can be characterized precisely from the point of view of body shape and pseudopodia, which researchers studying this group constantly utilize; for example, for amoebas of the genus Limax, an elongated body shape is characteristic, having some similarity to the shape of slugs, while for Amoeba radiosa it is star-shaped, etc. The phenomena of symmetry are very interesting in that they reveal a connection between form and way of life (ecological conditions), particularly in those traits that are often the most fundamental in the organization of a given group and have the greatest taxonomic significance—traits of types, classes. Organisms that with all points of their body come into contact with a homogeneous environment most often have a spherical shape, and repeating parts of the body are arranged along radial directions. Planktonic organisms, passively carried by currents in the water column or actively moving but rotating at the same time, often have a spherical body with radially arranged appendages, for example, radiolarians (especially from the group Sphaeroida); a few planktonic foraminifera (for example, Globigerina) have spherical chambers of the shell. The spherical shape of the body is also found in intracellular parasites, for example, in coccidia; in parasites immersed in more or less homogeneous tissue of the host's body, for example, the bladder stage of Echinococcus; interestingly, in the latter case, daughter blisters arise on the wall of the mother's bladder in radial directions. Among the simplest organisms, besides the aforementioned planktonic forms, a good example of this type of symmetry can be provided by heliozoans (non-sessile), which often have a very regular spherical body shape with numerous radially arranged axopods. The encystment of protozoa, which has the significance of isolation, withdrawal from the environment, is usually accompanied by rounding of the body, reducing its surface of contact with the environment to a minimum. This type of symmetry can be called equiaxial, or homaxonic, as it is characterized by the presence of many identical axes of symmetry. If an organism transitions to a sessile way of life, acquiring for example a stalk by means of which it attaches to a substrate, then its form of symmetry changes: the axis of symmetry must now pass through the stalk and therefore it can only be one; this is the uniaxial, or monaxonic, type of symmetry. Ecologically, attachment to a substrate means that the pole by which the organism is attached will experience different influences from the environment than the rest of the body, while the latter will experience identical influences from all lateral sides. The same result—the transformation of equiaxiality into uniaxiality—can also be obtained in another way, namely, if the organism transitions from passive movement in water to active swimming, always with the same end of the body forward. In this latter case, the anterior pole of the body experiences different environmental influences than the opposite pole, first coming into contact with various changes in the external environment encountered during movement; thus the anterior and posterior ends of the body become differentiated. In a sessile way of life, accordingly, the pole by which the organism is attached and the opposite one become differentiated—at animals usually with the mouth opening, and this pole is called oral, while the opposite, attached one is called aboral. Examples of uniaxial organisms that do not show differentiation into right and left, as well as dorsal and ventral sides of the body, and can be divided into two symmetrical halves by any plane passing through the axis, can be: oval or spindle-shaped free-swimming flagellates with organs of movement at one (anterior) end of the body; sessile flagellates (choanoflagellates), shelled rhizopods like difflugia and arcella, semi-sessile (Stentor) and sessile (Vorticella) infusoria and suctorians; from Metazoa—cup-shaped sponges. This type of symmetry can be called polysymmetrical due to the presence of an indefinitely large number of symmetry planes; it is mainly found among poorly differentiated, low-organized forms. Much more often in a sessile uniaxial animal, the organs repeating in radial directions become differentiated, the number of which is more or less limited; the number of symmetry planes is in this case determined by the number of repeating organs, and the polysymmetrical type of uniaxial symmetry passes into the radial symmetry type, very widespread among sessile animals. Entire types are characterized by radial symmetry: thus, for coelenterates, four-rayed symmetry is most characteristic, to a lesser degree six-rayed; for echinoderms, which apparently originated from bilaterally symmetrical forms, the repetition of organs in a quantity of five or multiples of five is typical. The radial symmetry of coelenterates in many cases passes into two-rayed symmetry due to the flattening of the pharyngeal tube and the transformation of the round mouth opening into a slit-like one (coral polyps), the development of two tentacles on opposite sides of the body (ctenophores), etc. The transition from radial symmetry to bilateral symmetry or to bilaterality (see), characteristic of most three-layered animals, apparently occurred in connection with the transition from a sessile way of life to crawling on a substrate. In this transition, as well as in some other cases already mentioned above, the anterior and posterior ends of the body became differentiated, but in addition other important changes occurred simultaneously. The mouth, which in low-mobility bottom organisms as a rule faces downward, toward the substrate, moved to the anterior end of the body; the ventral side apparently became the one that was heavier, largely due to the concentration of nutritive yolk on it in the embryo, while the dorsal side became the one that, as a result of the cleavage of eggs with differentiated animal and vegetative poles, gives rise to ectoderm with its sensory organs; the ventral side is usually flattened, while the dorsal side is more convex. As a result of these changes, the right and left sides of the body become distinguishable, and the single plane of symmetry divides the body into right and left halves. The transition to bilateral symmetry should not however be connected with the origin of bilaterally symmetrical multicellular animals, as it undoubtedly occurred repeatedly; among the simplest organisms crawling forward with the same end of the body, we find clearly bilaterally symmetrical forms like gregarines and ventricous infusoria; secondary transitions to bilaterality also exist among echinoderms in irregular sea urchins, holothurians, in connection with the method of movement. Sharp and constant deviations from the type of symmetry characteristic of any group, the phenomena of asymmetry (see), can also often be connected with ecological conditions. Bilaterality is violated if the right and left sides are connected with different environments: bivalve mollusks, attaching to the substrate with one of the valves (for example, Pecten), flatfish, lying on the bottom on one side, can serve as examples. The asymmetry of internal organs in snakes or the asymmetrical twists of the intestines, arising as a result of the elongation of the digestive canal, are related to the ecological conditions only indirectly. In other cases, for example in the case of asymmetry in snails, which is a characteristic feature of an entire class of mollusks, we do not find a clear ecological interpretation of the phenomenon. The ontogenetic development of symmetry often shows a replacement of primitive forms of symmetry with more specialized ones. Many eggs and blastulae are equiaxial, gastrulae, gastrula-like larvae, buds arising in asexual reproduction of coelenterates and sponges are polysymmetrical. Asymmetrical structure usually develops from normal symmetrical. Bilateral symmetry is generally determined early, but even earlier the axis of symmetry is outlined as the main axis of the egg, often connecting the animal pole with the vegetative one; it becomes the axis of the gastrula. In some chordate animals, bilateral symmetry appears after fertilization, while in some mollusks and arthropods the egg already shows bilateral symmetry. Among snails, there is a twisting of the visceral sac and the shell covering it in an unusual (left) direction; in such cases, the cleavage into the first two blastomeres proceeds differently, in the opposite direction to the usual one. In Lymnaea rubella, the direction of the coil is determined by one pair of allelic genes, with the gene for right coil being dominant. The connection of the type of symmetry with the conditions of existence is realized and consolidated in phylogenetic development as a result of natural selection and the survival of forms whose symmetry characteristics are most adapted to the given environmental conditions.