Abstract
Amphibia which are usually considered as a separate order, the Aistopoda. (2) The caecilians are limbless, long-bodied, extant, largely terrestrial amphibians. (3) The remaining attentuate amphibians are placed in the Urodela. Here complete limblessness is unknown. Representatives include the nearly limbless Amphiuma, in which both the front and hind limbs, though present, are very small. Pseudobranchus and Siren are unique aquatic forms that possess only a pair of tiny, anterior appendages. Though the relationships of the genera are uncertain, they are probably more distantly related to the salamanders than is generally presumed. The remaining nearly limbless salamander, Proteus, is an aquatic, subterranean genus of uncertain ancestry. The muscular system of the attentuate am? phibians is based on the primitive segmental arrangement of the fishes, in which each metameric unit is served by a pair of spinal nerves. Sheets of connective tissue, or myo? septa, separate each of the respective body segments. In amphibians, where muscles are sometimes constructed of components from several segments, the myosepta may still remain as evidence of this primitive segmentation.
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