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The Copepoda, the Ostracoda, and the Branchiopoda are the Crustacea which have departed least from the embryonic or Nauplius-forms; and, of these, I imagine that the Copepoda represent the hypothetical Archicarida most closely. Apus and Sapphirina indicate the relations of these Archaeocarids with the Trilobita, and the Eurypterida connect the Trilobita and the Copepoda with the Xiphosura.
But the Xiphosura have such close morphological relations with the Arachnida, and especially with the oldest known Arachnidan, Scorpio, that I cannot doubt the existence of a genetic connection between the two groups. On the other hand, the Branchiopoda do, even at the present day, almost pass into the true Podophthalmia, by Nebalia.
How well warranted are we therefore in identifying the latter with the former. Equally scanty is our knowledge of the developmental history of the Ostracoda. The development of Cypris has recently been observed by Claus: "The youngest stages are shell-bearing Nauplius-forms." The section of the Branchiopoda includes two groups differing even in their development, the Phyllopoda and the Cladocera.
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