IV CLASSIFICATION reg 
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 (G4) are present on the back. Both families 
possess a heart ; the Halocypridae are blind, while the Cyprid- 
inidae 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. 
