The Ostracoda are small Crustacea, the body consisting of very few—about eight—segments, and being completely enclosed in a carapace, which has the form of a bivalve shell. Development is direct, without a Nauplius stage.
The Ostracoda[[82]] are marine and fresh-water animals that can be divided into several families, differing slightly in habits and in structures correlated with those habits.
Fig. [74].—Candona reptans. A, Natural size; B, X 15. a, 1st antennae; b, 2nd antennae; c, walking legs. (After Baird.)
The Cypridae and Cytheridae include all the fresh-water and a vast majority of marine genera, adapted for a sluggish life among water-plants, though some can swim with considerable activity. The common Cypris and Candona of our ponds and streams are familiar instances. The movements of these animals are effected by means of the two pairs of uniramous pediform antennae which move together and in a vertical straight line. In the Cypridae (Fig. [74]) there are, besides the mandibles, two pairs of maxillae, a pair of walking legs, and, lastly, a pair of appendages, which are doubled up into the carapace, and are used for cleaning purposes. In the marine Cytheridae there is only one maxilla, the last three appendages being pediform and used in walking. The telson in the Cytheridae is rudimentary, but is well developed in the Cypridae. The heart is altogether absent.
In many of the fresh-water forms, e.g. common species of Candona and Cypris, males are never found, and parthenogenetic reproduction by the females appears to proceed uninterruptedly. Weismann[[83]] kept females of Cypris reptans breeding parthenogenetically for eight years. He also remarks on the fact that these, and indeed all parthenogenetic female Ostracoda, retain the receptaculum seminis, used normally for storing the spermatozoa derived from the male, unimpaired.
Some of the Cytheridae occur in deep water. Thus Cythere dictyon was frequently taken by the Challenger in depths of over 1000 fathoms, but the majority prefer shallow water.
Fig. [75].—Asterope oblonga, ♀, removed from its carapace, × 25. A, Alimentary canal; A1, A2, 1st and 2nd antennae; E, eye; G, gills; G.O, generative opening; H, heart; M, mandible; T, 6th appendage; T′, last appendage (cleaning foot). (After Claus.)
The Halocypridae and Cypridinidae comprise marine genera of a pelagic habit. The first antennae are chiefly sensory, but the second antennae are biramous, and they do not merely move up and down, as in the preceding families, but sideways like oars, the valves of the shells being excavated to admit of free movements. There are two pairs of maxillae; the succeeding limbs differ in the two families. In the Cypridinidae, e.g. Asterope (Fig. [75]), the first leg (T) is lamelliform and is used as an accessory maxilla, while the second leg (T’) is turned upwards into the shell as a cleaning organ. In the Halocypridae the first leg is pediform, and differs in the two sexes, while the second leg is rudimentary and points backwards. In Asterope peculiar branchial organs (G) are present on the back. Both families possess a heart; the Halocypridae are blind, while the Cypridinidae possess eyes.