Certain Cirripedes, viz. certain species of _Scalpellum_ and _Ibla_ and all the Acrothoracica, are dioecious. It is impossible to decide at present whether these species retain the primitive dioecious condition of the ancestral Cirripedes, or whether they too have been derived from an hermaphrodite state, but in the present state of knowledge they hardly affect the validity of the theory that has been proposed to account for the nature of the complemental males and the hermaphrodite individuals. =Order IV. Ostracoda.= 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. [Illustration: 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. [Illustration: FIG. 75.—_Asterope oblonga_, ♀, removed from its carapace, × 25. _A_, Alimentary canal; _A_{1}_, _A_{2}_, 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.) ]