Features common to mandibulates and all other arthropods
(A-19 through A-21) include a cuticle containing chitin and
protein. The adult arthropod coelom is very small. The principle
body cavity is a hemocoel—a cavity in which blood pumped
by the tubular heart circulates; from the hemocoel, blood enters
the heart through slit-shaped ostia characteristic of arthropod
hearts. Food is moved through the digestive tract by muscle
action, and nutrients are distributed by the blood. Most mandibulates
are terrestrial arthropods with internal gas exchange;
through thin-walled tracheae—minute tubes that penetrate
their tissues—body activity moves the gases. Air enters tracheae
through pores in the cuticle (spiracles), passing through hairs
that filter out small particles. Many species can close their spiracles,
retarding moisture loss. The mandibulate nervous system is
typically arthropod—a dorsal brain, ventral nerve cord ganglionated
in each segment, and compound or single-unit (simple;
that is, nonimage forming) eyes or both. Jointed appendages
and segmented bodies characterize all mandibulates and chelicerates
as arthropods. An insect has a single pair of antennae; a
crustacean has two pairs of antennae (Figures A and B), and a
chelicerate lacks antennae. A comparison of body parts reveals
that most mandibulates have head, thorax, and abdomen; the
chelicerate has a cephalothorax and abdomen. Mandibulates
have mandibles (Figure C)—mouthparts that crush food; chelicerates
lack mandibles. Here phylum Mandibulata includes
insects, myriapods, and Crustacea.