Description
They are mostly 0.2–6.0 millimetres (0.008–0.236 in) long, with the exception of Leptodora, which can be up to 18 mm (0.71 in) long. The body is not obviously segmented and bears a folded carapace which covers the thorax and abdomen.
The head is angled downwards, and may be separated from the rest of the body by a "cervical sinus" or notch. It bears a single black compound eye, located on the animal's midline, in all but two genera, and there is often a single ocellus. The head also bears two pairs of antennae – the first antennae are small, unsegmented appendages, while the second antennae are large, segmented and branched, with powerful muscles. The first antennae bear olfactory setae, while the second are used for swimming by most species. The pattern of setae on the second antennae is useful for identification. The part of the head which projects in front of the first antennae is known as the rostrum or "beak".
The mouthparts are small, and consist of an unpaired labrum, a pair of mandibles, a pair of maxillae, and an unpaired labium. They are used to eat "organic detritus of all kinds" and bacteria.
The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae. Carbon dioxide is lost, and oxygen taken up, through the body surface.
Read more about this topic: Cladocera
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