Bilaterality

BILATERALITY (from Latin bis-twice and latus-side), or bilateral symmetry, is a body structure in animals in which the body consists of identical halves—right and left. Bilaterality is characteristic of the majority of animal phyla (worms, mollusks, arthropods, vertebrates). Apparently, this body structure is better adapted to the performance of life functions than any other, judging by the fact that it is the most widespread among animals, and also because among animals built otherwise, transitions to bilaterality are found in the form of secondary adaptation (in sea urchins, holothurians, etc.). In bilaterality, body organs are either paired or fuse along the midline of the body, retaining their internal bilaterally symmetrical structure (nose, mouth, tongue, penis, vagina, uterus); unpaired organs, such as the heart, originate symmetrically as two tubes or, like the intestinal canal, as a single tube symmetrical with respect to the axis of the median plane. The symmetry of the structure and arrangement of various organs can reach mathematical symmetry, as, for example, in the arrangement, growth, and molting of bird feathers. One of the factors of bilateral symmetry is the uniform nutrition of both sides, as a result of which only organs not lying along the midline of the body (liver, spleen, intestine) are supplied by unpaired vessels (celiac, hepatic, and lienal arteries); whereas organs that are unpaired yet lie along the midline (pharynx, trachea, uterus, urinary bladder, penis) are supplied by paired vessels. The innervation of organs is also mostly symmetrical. In the process of evolution, bilateral structure may, as an adaptation to the external environment, pass into a bilaterally asymmetrical one, in which organs of one side disappear, become underdeveloped, or, conversely, greatly increase. In some crabs (Alpheus and others), one claw is several times larger than the other; in hermit crabs, which live in shells and close the entrance to them with the anterior part of their body, the right side is larger than the left. The incipient enhancement of one side over the other leads to the complete disappearance of the corresponding organ of the other side; thus, in some gastropod mollusks, initially paired gills or kidneys develop only on one side; the paired ovary of birds that originates during development ultimately develops only on one side. Complete asymmetry is relatively rare, as, for example, in flatfishes, in which the initially symmetrical structure changes with age by the displacement of organs to one side (eyes, many bones, etc.).