The defining feature of class Crustacea is the possession of
two pairs of antennae on the head, distinguishing crustaceans
from all other arthropods. Many familiar animals belong to
the Crustacea class—among them are crabs, lobsters, brine
shrimp, and krill. The two pairs of jointed antennae of the crayfish
Cambarus are typical crustacean features; the presence of
antennae distinguishes crustaceans from chelicerates (A-20).
Biramous (branched) appendages (Figure E) distinguish crustaceans
from insects and other members of phylum Mandibulata
with their unbranched appendages. Biramous appendages have
two branches to each appendage (limb); the two branches connect
to a single base, and each of the two branches consists of
several jointed segments. The crustacean exoskeleton may be
as thick as a lobster or as thin and flexible as the outer covering
of a soft-shelled crab larva. We agree to place pentastomes,
formerly phylum Pentastoma, as subclass Pentastomida within
class Crustacea. Pentastomes, highly modified necrotrophs, are
regarded by specialists to possibly be crustaceans, similar to
branchiurans (marine crustaceans that infect fish skin). Crustacea
number about 50,000 species.