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Many are limbless, limb-reduced, or apodous, with a body plan consisting of a head and vertebral column, but no adjoining limbs such as legs or fins. Agnatha are limbless but may have preceded the evolution of vertebrate limbs, whereas numerous reptile and amphibian lineages – and some eels and eel-like fish – independently lost their limbs. Larval amphibians, , are also often limbless. No mammals or birds are limbless, but some feature partial limb-loss or limb reduction.
Reptiles have on a number of occasions evolved into limbless forms – , , and (limb loss in lizards has evolved independently several times, examples include the families Pygopodidae and Dibamidae and species of Anguis, Isopachys, and Ophisaurus). Several species of have tiny useless legs, such as pygopodids which retain rudimentary flaps. Contrarily, the worm lizard Bipes as its scientific name suggests has two stubby forelimbs which actually assist in digging similar to a mole. All other amphisbaenians have reduced or absent forelimb girdles.
Larval amphibians, tadpoles, are often limbless. Adult amphibians have also evolved limblessness multiple times – , Sirenidae (a clade of that are limbless except for atrophied front limbs), Amphiumidae (a clade of salamander with extremely atrophied limbs that appear non-functional) and at least three extinct groups (Aïstopoda, Lysorophia, and Adelospondyli).
There are no known limbless species of mammal or bird, although partial limb-loss and reduction has occurred in several groups, including Cetacea, sirenians, kiwis, and the extinct moa and elephant birds. The moa in particular are notable for having completely lost their wings, without even vestigial wings remaining outside their bodies. Despite its name, the finless porpoise has two fins.
In the case of limb loss during evolution, vestigial structures testify to this change (remains of the pelvis, rudimentary femur or Pelvic spur in Boa constrictor, Pythonidae and Typhlops). The evolutionary process of transforming quadrupedal lizards into legless forms results in three main characteristics: the regression of the limbs is carried out gradually, via the reduction in their size and the reduction in the number of phalanges or fingers; the multiplication of the vertebrae (up to 600 in some snakes) induces a lengthening and a gain in flexibility of the trunk; and the vertebral axis is homogenized from the neck to the cloaca, evoking an interminable ribcage.
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