Segment

SEGMENT (from Latin segmentum - segment, part), or metamere, a division of the body of an animal partitioned along the main axis into sections in which certain organs are repeated. Segmentation can be external, as for example in annelids or arthropods, in which individual segments are separated from one another by clearly visible constrictions with a thinner cuticle. In each such segment of an annelid, there is a separate section of the secondary body cavity (coelom), separated by transverse partitions from the cavities of neighboring segments; there is one pair of nerve ganglia forming part of the ventral nerve cord, one pair of excretory tubules (nephridia), segmental blood vessels, and so on. Segmentation can be uniform, "homonomous," as in many annelids or myriapods, in which all body segments are completely equivalent and only the head segments stand out noticeably by their special differentiation in connection with the position of the mouth at the anterior end of the body, the development of a series of sense organs here, the transformation of appendages into organs for grasping and grinding food, and the like. There are also known differences in the posterior body segments in connection with the position of the anus and the openings of the genital ducts. Segmentation is "heteronomous" in the case of a more far-reaching division of labor between various body segments, associated as always with different differentiation. In crustaceans and especially in insects, the body breaks down into 3 regions, each consisting of a complex of partly fused, definitely differentiated segments united by a common function. These regions, known by the names of the head, thorax, and abdomen, are usually clearly separated from each other and acquire very different structures. The head is the bearer of higher sense organs and a number of appendages transformed into a complex mouth apparatus. The thorax is associated with the locomotor apparatus—swimming or walking limbs, and wings. The abdomen contains the greater part of the digestive organs and reproductive organs. In vertebrates, segmentation is also heteronomous and, moreover, only internal. It manifests itself first of all in the division of the dorsal region of the embryonic mesoderm into a series of "primary segments" or somites, giving rise to the rudiments of the axial skeleton (sclerotomes), the rudiments of the parietal musculature (myotomes), as well as the segmentally located rudiments of the primary kidney tubules. In adult vertebrates, segmentation is most easily discovered in the structure of the axial skeleton, consisting of a metameric series of vertebrae associated with a similar series of ribs. The heteronomy of this segmentation is clearly expressed, especially in higher vertebrates, in the different differentiation of the vertebrae (and ribs) in various regions of the spine (cervical, thoracic, lumbar, sacral, and caudal). The musculature retains its metameric structure especially in lower vertebrates. In fishes, the parietal musculature is composed of two large lateral muscles consisting of a successive series of homogeneous "muscle segments" ("myomeres"). In land vertebrates in the adult state, only traces of this segmentation are preserved, mainly in the arrangement of the intervertebral and intercostal muscles. Closely related to muscle metamerism is the division of the nervous system of vertebrates into a series of nerve segments. In embryos, this division is expressed sometimes partially in the visible segmentation of the neural tube (neuromery), but mainly in the structure of the ganglionic plate associated with it. In adult vertebrates, only the peripheral nervous system is segmented, consisting of a series of spinal ganglia with segmental nerves extending from them.