General Considerations: MORPHOLOGY OF THE OSTRACODA; ANATOMICAL FEATURES
The minute bivalved crustaceans, known as Ostracoda, exist in countless numbers in both fresh and marine waters. Just as today, so in the past they were exceedingly prolific, certain rock strata being composed almost entirely of their shells and separated valves. The fossil forms, moreover, are constant in the lobing, surface ornamentation, and other features of their shells, so that they have become useful in identifying stratigraphic horizons.
The Ostracoda are small, generally minute, crustaceans, with the entire body inclosed in a horny or calcareous carapace, the right and left sides of which are separate and articulated along the dorsal edge so as to form a bivalved shell. The body is indistinctly segmented and has seven pairs of appendages of which the first two are antennae, which, like the others, are also adapted for creeping and swimming. These appendages, together with the caudal extremity of the abdomen, are protruded along the ventral margin of the carapace when the valves are opened.
Behind the first two pairs of appendages (antennules and antennae) is a pair of mandibles, followed by two pairs of maxillae, and finally by two pairs of slender legs. The abdomen is short and rudimentary, and its extremity may consist of a single spinous plate or may be bifurcated. The details of the anatomy of the animal are shown in Figure 1. With a single exception the fossil species preserve only the carapace valves (Fig. 1–1), so that the anatomy of the animal is known almost entirely from . . .