Copepods are typically small, with a body length in the range of 0.019-0.78 in (0.5-2 mm), although some free-living forms attain lengths of up to 0.7 in (18 mm), and some highly modified parasites can reach 7.8 in (20 cm) in length. The copepod body plan consists of two regions, the anterior prosome and the posterior urosome. The prosome comprises
the cephalosome, plus either four or five leg-bearing segments, according to group. The cephalosome is composed of the segments that carry the five pairs of head limbs typical of crustaceans, including antennules, antennae, mandibles, max-illules and maxillae, as well as the first thoracic segment carrying the maxillipeds. The six segments are covered by the single, dorsal head shield, and the median nauplius eye lies frontally, beneath the shield. Behind the cephalosome are the free prosomal segments with swimming leg pairs, either four or five of them. In the gymnoplean body plan, as found in Platycopioida and Calanoida, there are five leg pairs, whereas in the podoplean body plan, as found in all the other seven orders, there are only four pairs. The boundary between the prosome and the urosome is less distinct in the Harpacticoida, many of which have slender cylindrical bodies. The urosome is primitively four- or five-segmented depending on the group, and the last segment bears the anus and caudal rami. The most characteristic feature of copepods is the form of the swimming legs: members of each leg pair are fused to a median intercoxal sclerite, which ensures that left and right legs always beat together. The legs are two-branched (biramous) and each branch (endopod and exopod) consists of a maximum of three segments. Antennules can express up to 28 articulating segments in a single axis, but segment numbers are always less than this because of the failure of expression of one or more joints. The female typically carries the eggs in paired egg sacs, although many marine calanoids broadcast their eggs instead.
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