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. The =Polycopidae= and =Cytherellidae= are curious marine families of a pelagic habit, with biramous second antennae well adapted for swimming, and very broad. The first maxilla in the Polycopidae is also employed in swimming, while the second is modified into a branchial organ; the maxillae of the Cytherellidae are more normal in structure, but both carry branchial lamellae. The posterior limbs are altogether absent in Polycopidae, and in the Cytherellidae are only represented by the copulatory organs of the male. CHAPTER V CRUSTACEA (_CONTINUED_): MALACOSTRACA: LEPTOSTRACA—PHYLLOCARIDA: EUMALACOSTRACA: SYNCARIDA—ANASPIDACEA: PERACARIDA—MYSIDACEA—CUMACEA— ISOPODA—AMPHIPODA: HOPLOCARIDA—STOMATOPODA SUB-CLASS II.—MALACOSTRACA. The Malacostraca are generally large Crustacea, and they are characterised by the presence of a definite and constant number of segments composing the body. In addition to the paired eyes we can distinguish two pairs of antennae, a mandibular segment, and two maxillary segments composing the head region proper; there then follow eight thoracic segments, the limbs belonging to the anterior thoracic segments being often turned forwards towards the mouth, and modified in structure to act as maxillipedes, while at any rate the last four are used in locomotion and are termed “pereiopods.”[84] The abdomen is composed of six segments, which typically carry as many pairs of biramous “pleopods,” and the body terminates in a telson. Not counting the paired eyes or the telson, there are present nineteen segments. The excretory organs in the adult open at the bases of the second antennae, and are known as “green glands,” but in the larva maxillary glands may be present homologous to those which persist in the adult Entomostraca. This is the typical arrangement, but sometimes the maxillary glands persist in adult Malacostraca, _e.g._ _Nebalia_, _Anaspides_, and some Isopods.