![]() In summary, this order has many morphological simplifications or losses as a result of their evolutionary trend toward a worm-like configuration the increased number of vertebrae is the result of the same phenomenon. They lack both pyloric caeca and oviducts but have retained the swim bladder. The skeleton is reduced, but the vertebrae may number as many as 700. ![]() These species also have lost gill rakers. The gill openings usually are narrow, with the gill region elongated and the gills displaced posteriorly. Most species also lack scales in those species that have them, they are cycloid in type and embedded under the skin. Most species do not have pectoral fins, but when they are present, they lack bony connections to the skull. Systematists have emphasized numerous other morphological characteristics that have been found useful for phylogenetic purposes, including the lack of pelvic fins and the continuous dorsal, anal, and caudal fins that can have up to 650 soft rays, giving some individuals the appearance of having a pointed tail. Adult sizes range from about 4 in (10 cm) to 11.5 ft (3.5 m), as in the moray species Thyrsoidea macrura. In addition to their eel-like bodies, anguilliform species have widely varying coloration that ranges from black or dark gray in deep-sea species to rich colors and complex patterns in tropical reef species. Much more work is needed in this area to determine the exact phylogenetic relationships within this group. Nelson divided this order into three suborders and 15 families (Anguillidae: 15 spp. Some researchers, such as Filleul and Lavoué, have questioned this phylogenetic relationship based on molecular studies. ![]() The larval stage groups them into the subdivision or superorder called Elopomorpha. Anguilliformes are related to the Elopiformes (tarpons), the Albuliformes (spiny eels and halosaurs), and the Saccopharyngiformes (snipe and gulper eels) because they all have a leptocephalus, or ribbonlike, larval stage during development. The Anguilliformes also are called Apodes ("limbless"), because of their lack of protruding fins, and true eels, because there are many other fishes (about 45 families) that do not belong to this group but have similar burrowing habits, and an eel-like shape as a result of convergent evolution. Fossil Anguilliformes are known from the Upper Cretaceous (about 93 million years ago) until the Pliocene (about two million years ago) and have been found in Africa, Europe, North America, the East Indies, Australia, and New Zealand.
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