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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
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]
Arthropod heads are typically fused capsules that bear a variety of complex structures such as the eyes, antennae and mouth parts. The challenge that the arthropod head problem has to address is to what extent the various structures of the arthropod head can be resolved into a set of hypothetical ancestral segments.
Transverse section of a human embryo of the third week to show the differentiation of the primitive segment. ao. Aorta. m.p. Muscle-plate. n.c. Neural canal. sc. Sclerotome. s.p. Dermatome
Segmentation (biology) – Division of some animal and plant body plans into a series of segments; Supernumerary body part – Growth of an additional part of the body and a deviation from the body plan; Symmetry in biology – Geometric symmetry in living beings
Arthropods (/ ˈ ɑːr θ r ə p ɒ d / ARTH-rə-pod) are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin , often mineralised with calcium carbonate , a body with differentiated ( metameric ) segments , and paired jointed appendages .
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.
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).