Fig. 11—Estheria obliqua, One of the Conchostraca. (After Sars, from Lankester's "Treatise on Zoology.")
A, Shell of female, from the side; B, male, from the side, after removal of one valve of the shell. (Enlarged.) a′, Antennule; a″, antenna; ad, muscle which draws together the valves of the shell; f, tail fork; md, mandible
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Fig. 12—Daphnia pulex, a Common Species of "Water-flea." Much enlarged. (From British Museum Guide.)
Female carrying eggs in the brood-chamber
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The third order of the Branchiopoda, the Conchostraca ([Fig. 11]), are not represented in Britain, though several species occur on the Continent of Europe. In these the carapace forms a bivalved shell, completely enclosing the body and limbs, and closely resembling that of a small Mollusc.
The fourth order, the Cladocera, comprises the so-called "Water-fleas," which are abundant everywhere in ponds and lakes ([Fig. 12]). They are all of small size, almost or quite microscopic. The carapace, as in the Conchostraca, forms a bivalved shell, but does not enclose the head. There is a single large eye, which really corresponds to two eyes fused together. A pair of large antennæ, each with two branches, carrying long feathered hairs, project at the sides of the head, and are used in swimming with a peculiar jumping motion, from which the popular name of the animals is derived. There are not more than six pairs of feet. The "Water-fleas," of which Daphnia pulex is one of the commonest species, are very beautiful and interesting objects for microscopic examination, on account of their transparency, which allows many details of their internal structure to be studied in the living animal.