Amphionids are marine crustaceans with a number of unique, identifying characteristics. An adult amphionid is about 1 in (2.6 cm) long. The head and thorax are covered by a thin, shell-like carapace that encloses the dorsal and ventral surfaces and about half the animal's total length, with considerable space between the ventral body surface and the bottom of the carapace. There are gaps in the front and rear of the carapace. The thorax and abdomen are distinct and differentiated.
The head bears a pair of stalked, multifaceted eyes, the stalks each carrying a structure that is probably a photophore, or light-producing organ. There are two sets of biramous (two-branched) antennae, each antenna of the second pair bearing large, leaf-shaped accessory structures, the antennal scales, or scaphocerites. There are seven pairs of thoracic appendages, the paraeopods, all but the seventh being biramous. Only the first pair, close to the mouth, is functional, the max-illipeds being used for swimming. There is a wide gap between the first and second pair of appendages. The second to sixth pairs are thin and sticklike. They are useless for swimming and probably are not remnants of true maxillipeds. In the female, the fifth thoracic paraeopods are much longer than the rest, while the coxae, or proximal segments of the sixth pareaopods, carry genital pores. The seventh thoracic segment lacks appendages in the female, but bears a pair of uni-ramous (unbranched) appendages in juvenile males. Pairs of feathery pleurobranchs (gills) are mounted on the third to seventh thoracic somites (segments).
The heart, placed dorsally in the rear part of the thorax, is well developed. In females, ovaries run from the heart region to oviducts that lead to the genital pores in the sixth thoracic appendages.
Two pairs of longitudinal muscles link the posterior thorax and the abdomen. The abdomen divides into six somites, of which five bear appendages, the pleopods. In the female, the pleopods on the first abdominal segment are very long, thin, and flattened, able to reach forward to about half the length of the carapace. When extended forward, these specialized appendages are able to cover the rear and part of the forward gap of the carapace, thus rendering it a brood chamber. The unusually long fifth thoracic pareaopods may serve to clean and tend the eggs. In the male, judging by the larval stages, the first pair of pleopods is stoutly constructed and capable of vigorous propulsion.
Following the first pair of pleopods are four more pairs of shorter, biramous pleopods. The last segment carries a pair of uropods, or tail appendages.
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