Tagma (biology)

: different kinds of segment are joined together into tagmata. Two thoracic segments are fused into the head; one thoracic segment is in the posterior tagma. Other kinds of copepod also have two tagmata but formed by different segments. The development of distinct tagmata is believed to be a feature of the evolution of segmented animals, especially arthropods. In the ancestral arthropod, the body was made up of repeated segments, each with similar internal organs and appendages. One evolutionary trend is the grouping together of some segments into larger units, the tagmata. The evolutionary process of grouping is called tagmosis (or tagmatization). The first and simplest stage was a division into two tagmata: an anterior "head" (cephalon) and a posterior "trunk". The head contained the brain and carried sensory and feeding appendages. The trunk bore the appendages responsible for locomotion and respiration (gills in aquatic species). In almost all modern arthropods, the trunk is further divided into a "thorax" and an "abdomen", with the thorax bearing the main locomotory appendages. In some groups, such as arachnids, the cephalon (head) and thorax are hardly distinct externally and form a single tagma, the "cephalothorax" or "prosoma". Mites appear to have a single tagma with no obvious external signs of either segments or separate tagmata. Tagmosis proceeded differently in different groups of arthropods, so that the tagmata are not derived from corresponding (homologous) segments, even though the same names may be used for the tagmata. Copepods (a kind of crustacean) provide an example. The basic copepod body consists of a head, a thorax with six segments, ancestrally each with a swimming leg, and an abdomen with five appendageless segments. Except in parasitic species, the body is divided functionally into two tagmata, that may be called a "prosome" and a "urosome", with an articulation between them allowing the body to flex. Different groups of copepods have the articulation at different places. In the Calanoida, the articulation is between the thoracic and abdominal segments, so that the boundary between the prosome and urosome corresponds to the boundary between thoracic and abdominal segments. However, in the Harpacticoida, the articulation is between the fifth and sixth thoracic segments, so that the sixth thoracic segment is in the urosome (see the diagram). Tagmosis is an extreme form of heteronomy, mediated by Hox genes and the other developmental genes they influence. ==Terminology==

Trilobite sections-en.svg|The tagmata of a trilobite: cephalon, thorax and pygidium File:Tagma.png|Tagmata of an insect: head (cephalon), thorax and abdomen Tagmata and major appendages of spider IMG 7639A.JPG|Tagmata and major appendages of a spider: cephalothorax or prosoma and abdomen or opisthosoma The number of tagma and their names vary among taxa. For example, the extinct trilobites had three tagmata: the cephalon (meaning head), the thorax (literally meaning chest, but in this application referring to the mid-portion of the body), and the pygidium (meaning rump). The Hexapoda, including insects, also have three tagmata, usually termed the head, thorax, and abdomen. The bodies of many arachnids, such as spiders, have two tagmata, as do the bodies of some crustaceans: in both groups the anterior tagma may be called the cephalothorax (meaning head plus chest) or the prosoma or prosome (meaning "fore-part of body"). The posterior tagma may be called the abdomen. In those arachnids that have two tagmata, the abdomen is also called the opisthosoma. In crustaceans, the posterior tagma is also called the pleon or the urosome (meaning the tail part); alternatively, "pleon" may refer only to the abdominal segments incorporated into the posterior tagma, the thoracic segments in this tagma being called the "pereon". ==See also==