What Is Tagmatization And Why Is It Advantageous For Metameric Animals

In biology, a tagma (Greek: τάγμα, plural tagmata – τάγματα - body of soldiers; battalion) is a specialized grouping of multiple segments or metameres into a coherently functional morphological unit of measurement. Familiar examples are the caput, the thorax, and the belly of insects.^[1] The segments inside a tagma may be either fused (such as in the head of an insect) or so jointed as to exist independently moveable (such as in the abdomen of most insects).

Usually the term is taken to refer to tagmata in the morphology of members of the phylum Arthropoda, but it applies equally validly in other phyla, such equally the Chordata.

In a given taxon the names assigned to particular tagmata are in some sense informal and arbitrary; for example, not all the tagmata of species within a given subphylum of the Arthropoda are homologous to those of species in other subphyla; for ane affair they do not all contain respective somites, and for another, not all the tagmata accept closely coordinating functions or anatomy. In some cases this has led to earlier names for tagmata being more or less successfully superseded. For example, the i-time terms "cephalothorax" and "belly" of the Araneae, though not yet strictly regarded as invalid, are giving way to prosoma and opisthosoma. The latter two terms carry less of a proffer of homology with the significantly different tagmata of insects.

Tagmosis [edit]

Tagmata in Harpacticoida: unlike kinds of segment are joined together into tagmata. Ii thoracic segments are fused into the head; one thoracic segment is in the posterior tagma. Other kinds of copepod also take two tagmata only formed by dissimilar segments.

The development of singled-out tagmata is believed to be a feature of the evolution of segmented animals, peculiarly arthropods. In the ancestral arthropod, the body was made up of repeated segments, each with similar internal organs and appendages. I evolutionary trend is the grouping together of some segments into larger units, the tagmata. The evolutionary process of group is called tagmosis (or tagmatization).^[2]

The starting time and simplest stage was a division into two tagmata: an anterior "caput" (cephalon) and a posterior "trunk". The caput contained the brain and carried sensory and feeding appendages. The body bore the appendages responsible for locomotion and respiration (gills in aquatic species). In almost all mod arthropods, the body is farther divided into a "thorax" and an "belly", with the thorax bearing the main locomotory appendages. In some groups, such equally arachnids, the cephalon (head) and thorax are hardly distinct externally and class a unmarried tagma, the "cephalothorax" or "prosoma". Mites appear to have a single tagma with no obvious external signs of either segments or separate tagmata.^[2]

Tagmosis proceeded differently in dissimilar groups of arthropods, then that the tagmata are not derived from corresponding (homologous) segments, even though the aforementioned names may exist used for the tagmata.^[three] Copepods (a kind of crustacean) provide an example. The basic copepod torso consists of a caput, a thorax with six segments, ancestrally each with a pond leg, and an abdomen with five appendageless segments. Except in parasitic species, the body is divided functionally into two tagmata, that may exist called a "prosome" and a "urosome", with an articulation between them allowing the trunk to flex. Different groups of copepods have the joint at unlike places. In the Calanoida, the articulation is between the thoracic and intestinal segments, so that the boundary between the prosome and urosome corresponds to the boundary betwixt thoracic and intestinal segments. However, in the Harpacticoida, the articulation is betwixt the fifth and sixth thoracic segments, so that the sixth thoracic segment is in the urosome (see the diagram).^[iv]

Tagmosis is an extreme grade of heteronomy, mediated by Hox genes and the other developmental genes they influence.^[v]

Terminology [edit]

The tagmata of a trilobite: cephalon, thorax and pygidium
Tagmata of an insect: head (cephalon), thorax and belly
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 case, the extinct trilobites had three tagmata: the cephalon (meaning head), the thorax (literally significant breast, simply in this awarding referring to the mid-portion of the body), and the pygidium (meaning rump). The Hexapoda, including insects, also have iii tagmata, usually termed the caput, 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 torso"). The posterior tagma may be called the abdomen. In those arachnids that have two tagmata, the belly is as well chosen the opisthosoma. In crustaceans, the posterior tagma is also chosen the pleon or the urosome (meaning the tail part); alternatively, "pleon" may refer just to the abdominal segments incorporated into the posterior tagma, the thoracic segments in this tagma beingness called the "pereon".^[6]

References [edit]

^ D. R. Khanna (2004). Biological science of Arthropoda. Discovery Publishing House. ISBN978-81-7141-897-eight.
^ ^a ^b Ruppert, Flim-flam & Barnes (2004), pp. 518–520.
^ Ruppert, Fob & Barnes 2004, p. 518.
^ Barnes et al. (2001), p. 198.
^ Alessandro Minelli (2003). "Body Regions: Their Boundaries and Complexity". The development of animal form: ontogeny, morphology, and evolution. Cambridge University Press. pp. 79–105. ISBN978-0-521-80851-iv.
^ Barnes et al. (2001), p. 191.

Bibliography [edit]

Barnes, R.S.One thousand.; Calow, P.; Olive, P.J.W.; Golding, D.W. & Spicer, J.I. (2001). The Invertebrates: a Synthesis (tertiary ed.). Wiley-Blackwell. ISBN978-0-632-04761-1.
Ruppert, E.E.; Fox, R.Southward. & Barnes, R.D. (2004). Invertebrate Zoology (7th ed.). Brooks/Cole. ISBN978-0-03-025982-one.