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Heteronomous annulation is a characteristic of some arthropods. It means that segments are differentiated from one another, each having different form so as to serve a different function – for instance, using some limbs for walking and others for feeding. The trait is first observed in the armoured lobopods. [1]
Segmentation in animals typically falls into three types, characteristic of different arthropods, vertebrates, and annelids. Arthropods such as the fruit fly form segments from a field of equivalent cells based on transcription factor gradients. Vertebrates like the zebrafish use oscillating gene expression to define segments known as somites
The arthropod head problem has been tackled in three main ways in this regard, first by using genetic segmental markers to probe the obscure region in front of the mouth, especially in insects; second by looking at Hox gene expression patterns to detect patterns of homology among different arthropods; and third, by studying gene expression in ...
As in all arthropods, the chelicerate body has a very small coelom restricted to small areas round the reproductive and excretory systems. The main body cavity is a hemocoel that runs most of the length of the body and through which blood flows, driven by a tubular heart that collects blood from the rear and pumps it forward.
The arthropod body plan consists of segments, each with a pair of appendages. Arthropods are bilaterally symmetrical and their body possesses an external skeleton . In order to keep growing, they must go through stages of moulting , a process by which they shed their exoskeleton to reveal a new one.
A segmentation gene is a gene involved in the early developmental stages of pattern formation. It regulates how cells are organized and defines repeated units in the embryo. Segmentation genes have been documented in three taxa: arthropods (i.e. insects and crabs), [2] chordates (i.e. mammals and fish), and annelids (i.e. leeches and earthworms).
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.
In chelicerates and crustaceans, the cephalothorax is derived from the fusion of the cephalon and the thorax, and is usually covered by a single unsegmented carapace.In relation with the arthropod head problem, phylogeny studies show that members of the Malacostraca class of crustaceans have five segments in the cephalon, when not fused with the thorax to form a cephalothorax.