Physical characteristics

Cladocerans are small animals that are often shaped like flat disks. Most range in size from 0.008 to 0.1 in (0.2-3 mm), although one species, Leptodora kindtii can grow as large as 0.7 in (18 mm) in length. Their bodies are not clearly segmented like those of other crustaceans; however, three parts can be distinguished—head, thorax, and abdomen. The head is typically dome-shaped with large compound eyes and five pairs of appendages. Among the appendages are two pairs of antennae—a small pair that serves a sensory function and a larger pair that is used for swimming. The other three pairs of appendages function in securing food.

A thin transparent shell called the carapace encloses the thorax and abdomen of most cladocerans. The carapace may be patterned; some species have stiff spines protruding from their shells. The thorax holds four to six pairs of leaflike legs that are used for gathering food, filtering water, or grasping mates during copulation. The cladoceran digestive tract is contained within the thorax and abdomen. A pair of claws used for cleaning the thoracic legs may extend from the end of the carapace.

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