+Branchiopoda+ | | | | | | | | | | | | | | \----------------------v-------------------------/ | Nauplius | +Archicaridæ+ (Articulated Worms) The pedigree on p. 177 will show how we may at present approximately conceive the derivation of the twenty orders of Crustacea enumerated on p. 176, from the common primary form of the Nauplius. Out of the Nauplius form—which originally existed as an independent genus—the five legions of lower Crabs developed as diverging branches in different directions, which in the systematic survey of the class are united as Segmented Crabs (Entomostraca). The higher division of Mailed Crabs (Malacostraca) have likewise originated out of the common Nauplius form. The Nebalia is still a direct form of transition from the Phyllopods to the Schizopods, that is, to the primary form of the stalk-eyed and sessile-eyed Mailed Crabs. The Nauplius at this stage gives rise to another larva form, the so-called Zoëa, which is of great importance. The order of Schizopoda, those with cloven feet (Mysis, etc.), probably originated from this curious Zoëa; they are at present still directly allied, through the Nebalia to the Phyllopoda, those with foliaceous feet. But of all living crabs the Phyllopods are the most closely allied to the original primary form of the Nauplius. Out of the Schizopoda the stalk-eyed and sessile-eyed Mailed Crabs, or Malacostraca, developed as two diverging branches in different directions: the former through shrimps (Peneus, etc.), the latter through the Cumacea (Cuma, etc.), which are still living and closely allied to the Schizopoda. Among those with stalked eyes is the river crab (cray-fish), the lobster, and the others with long tails, or the Macrura, out of which, in the chalk period, the short-tailed crabs, or Brachyura, developed by the degeneration of the tail. Those with sessile eyes divide into the two branches of Flea-crabs (Amphipoda) and Louse-crabs (Isopoda); among the latter are our common Rock-slaters and Wood-lice.