KEY TO CLASSES OF PHYLUM ARTHROPODA

1. a. Arthropods with body divided by two longitudinal furrows into three distinct lobes; appendages (2 per segment) are numerous and similar; now extinct, known only as fossils. (Subphylum Trilobita) --- Trilobite fossil
   b. Arthropods without the above combination of characteristics --- 2
2. a. Without jaws and antennae; first pair of ventral appendages are chelicerae (Subphylum Chelicerata) --- 3
   b. With jaws (mandibles) and antennae (Mandibulates) --- 5
3. a. Possessing abdominal appendages and a long spike like telson --- Class Merostomata
   b. Lacking the above characteristics --- 4
4. a. Body divided into cephalothorax and abdomen with a very large abdomen; four pairs of walking legs present on cephalothorax --- Class Arachnida
   b. Body divided into cephalothorax and abdomen with a very reduced abdomen; four to six pairs of walking legs present on cephalothorax --- Class Pycnogonida
5. a. Possessing gills, biramous appendages and 2 pairs of antennae --- Class Crustacea
   b. Possessing one pair of antennae and no gills in the adult form --- 6
6. a. Body divided into head, thorax and abdomen; 3 pairs of legs on thorax --- Class Insecta
   b. Body divided into head and trunk; numerous trunk appendages --- 7
7. a. Possessing one pair of appendages on each trunk segment; first pair of trunk appendages (maxillipeds) are poison claws --- Class Chilopoda
   b. Possessing 2 pair of appendages on each apparent trunk segment --- Class Diplopoda

adapted from material from Brown University