Grade x (below the Arthropoda).—Agnatha, Aprosthomera.
Without parapodial jaws; without the addition of originally post-oral somites to the prae-oral region, which is a simple prostomial lobe of the first somite; the first somite is perforated by the mouth and its parapodia are not modified as jaws.
= Chartopoda.
Grade 1 (of the Arthropoda).—Monognatha, Monoprosthomera.
With a single pair of parapodial jaws carried by the somite which is perforated by the mouth; this is not the first somite, but the second. The first somite has become a prosthomere, and carries a pair of extensile antennae.
= Onychophora (Peripatus, &c.).
Grade 2 (of the Arthropoda).—Dignatha, Monoprosthomera.
The third somite as well as the second develops a pair of parapodial jaws; the first somite is a prosthomere carrying jointed antennae.
= Diplopoda.