Bathynellacea range in length between 0.02 in (0.5 mm) and 0.14 in (3.5 mm). As an adaptation to their subterranean existence, they do not have eyes and lack body pigment. The body is divided into head, thorax, and abdomen. The head carries the antennae with chemo- and mechanosensory structures as well as the mouthparts. The mandibles, used for biting, are symmetrical in structure. The thorax has seven pairs of biramous walking legs. The eighth pair is reduced in both sexes and in the male is transformed into a characteristic cop-ulatory organ. The abdomen carries one pair of appendages on the first and last somite (pleotelson). There can also be a pair of appendages on the second abdominal somite. The pair on the last somite (uropods) does not form a tailfan together with the pleotelson, as is the case in several related groups of Crustacea. The basal segment (sympodite) of the uropods carries a characteristic row of spines. Bathynellidae can be identified by the second antennae being directed anteriorly, by the last abdominal somite (pleotelson) carrying two dorsal setae, by the rim of the upper lip (labrum) being smooth and not serrate, by the outer branches (exopodites) of all seven walking legs being one-segmented, by the first abdominal appendages (pleopods) being two-segmented, and by the sym-podite of the uropods being relatively short.
In contrast, Parabathynellidae have the second antennae bent backwards and carry lateral setae on the pleotelson. The rim of their labrum is serrate or fringed with fine setae. The exopodites of the walking legs are composed of one or more segments and only rarely do the exopodites of all the legs have the same number of segments. The first pleopods, if present, are one-segmented or represented by only two setae. The sympodite of the uropods is more elongate than in the Ba-thynellidae.
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