A reasonable degree of morphological diversity is found among the Atheriniformes, and, in particular, phallostethids exhibit a form that is unique among all fishes. The typical sil-verside generally is elongate and laterally compressed, with two dorsal fins, a single anal fin spine, usually cycloid scales, and no lateral line. Most are silvery in color and have a prominent mid-lateral stripe along each side of the body. Melano-taeniids are constructed similarly but often are deeper bodied and have sexually dimorphic color patterns. Male rainbow-fishes frequently are brilliantly colored, with complex patterns, in various shades of red, yellow, orange, blue, and green. In telmatherinids, as well as some pseudomugilids and melan-otaeniids, the anal and dorsal fins sometimes are elongated into filamentous extensions or elaborate fanlike shapes.
Most noteworthy among atheriniforms, however, are the extraordinarily modified phallostethids (priapium fishes). In both male and female phallostethids, the anal and genital openings are shifted anteriorly and are located under the throat. Males have an elaborate, bilaterally asymmetric cop-ulatory structure under the head, called a priapium, for which no parallel exists among other fishes. A suspensory component of the priapium is made up of modified anterior pleural ribs and pelvic bones. Emerging from the posterior end of the priapium and arching forward almost the entire length of the head are the ctenactinia—elongate, curved bones used for clasping the female during mating. In different species, the anus is offset to one or the other side of the priapium, and the seminal papilla is offset opposite the anus. Both open anteriorly. The seminal papilla, which is elaborated into a cop-ulatory organ in some species, is used to transfer bundles of sperm to the female.
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