Daphnia pulex is the most common species of the group of organisms known as water fleas. Their common name was given because of their general appearance and jerky swimming motions which resembles that of the land flea. They are, in reality, a type of small crustacean and are generally 0.2-3.0 mm long. Their bodies are not distinctly segmented, but an important feature of their anatomy is the carapace, a folded shell-like structure which covers the animal and opens both ventrally and posteriorly. Studying the anatomy of this organism is made easier by the fact that most of its outer covering is clear, showing most of the internal organs at work, including the heart. The head of the organism contains both a darkly colored compound eye and numerous antennae used for feeling and swimming. Many Daphnia, including D. pulex and D. magna have a specialized light-sensing organ similiar to a tiny eye called an ocellus. Located posteriorly at the junction of the head are small, hard to see mouthparts. They mainly consist of the mandibles which are in constant motion and used by the organism to crush and grind its food. In a live specimen food particles can be seen passing through the intestine which terminates at the anus located on the postabdomen. The postabdomen is the most posterior part of the body and terminates itself in two hooklike cuticular claws used by the organism to clear debris out of the carapace. The fine teeth located on these claws are often used for species identification. The central portion of the body is the thorax and contains four to six pairs of flattened legs covered in setae. Daphnia males are generally smaller than females but have longer antennules and a modified postabdomen. Daphnia females posses a brood chamber located between the body wall and dorsal surface of the carapace used to carry their eggs.
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