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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.
Species of Peneus live in the European seas, as well as here, and their Nauplius-brood has no doubt repeatedly passed unnoticed through the hands of the numerous naturalists who have investigated those seas, as well as through my own,* for it has nothing which could attract particular attention amongst the multifarious and often wonderful Nauplius-forms.
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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