Comprising the primitive phyllopods, the minute copepods, the parasitic cirrhipedes and the powerful crabs and lobsters, and the highly elaborated sand-hoppers and slaters.
The bi-ramose structure of the post-oral limbs, demonstrated by Beecher in the trilobite Triarthrus, is no more inconsistent with its claim to be a primitive Arachnid than is the foliaceous modification of the limbs in Phyllopods inconsistent with their relationship to the Arthrostracous Crustaceans such as Gammarus and Oniscus.
Sars (1887) having had the opportunity of raising it from dried Australian mud, found that, unlike other phyllopods, but like the Cladocera, the parent keeps its brood within the shell until their full development.
(Bairdia Carbonia), phyllopods (Estheria), phyllocarids (Acanthocaris, Dithyrocaris), schizopods (Anthrapalaemon), Eurypterids (Eurypterus, Glyptoscorpius).
It is most closely followed by the Phyllopods such as Apus or Branchipus, and by some Copepoda.
The opinion now most generally held is that the primitive Crustacean type is most nearly approached by certain Phyllopods such as Apus.
The Ostracoda might have been derived from the same stock were it not that they retain the mandibular palp which all the Phyllopods have lost.