The evolution theory . dvanced state, nearerto that of the perfect animal, towhich it can, therefore, attain ina shorter time. We should expect that thisshortening of the larval periodwould be associated with a pro-longation of embryogenesis, es-pecially in those Crustaceanswhich possess a large number ofsegments and limbs, that is—inthe higher forms—and in themain this is the case. But thereare exceptions in two directions ;in the first place there are some, •even among the lower Crusta-ceans, which leave the egg notas a nauplius but in the perfectform of the adult, and secondly, there are, a
The evolution theory . dvanced state, nearerto that of the perfect animal, towhich it can, therefore, attain ina shorter time. We should expect that thisshortening of the larval periodwould be associated with a pro-longation of embryogenesis, es-pecially in those Crustaceanswhich possess a large number ofsegments and limbs, that is—inthe higher forms—and in themain this is the case. But thereare exceptions in two directions ;in the first place there are some, •even among the lower Crusta-ceans, which leave the egg notas a nauplius but in the perfectform of the adult, and secondly, there are, among the higher Crustaceans,certain species which emerge from the egg not in the more mature form but still in the primitive nauplius form. Fritz Miiller was the first to furnish an example of this last case, a Brazilian shrimp, Peneus 2^0- tlinirim. Like the lowest Copepods or Branchiopods, this species, which belongs to the highest order of Crustaceans, goes through the whole long-development, from the nauplius. Fig. 109. C, second Zoaea sttige. Thethorax is now divided into cephalotliorax{Cph) and abdomen {Ahd) ; seven i)airs ofthrough a series of higher larval appendages are developed, and five inoi forms up to the perfect animal, |Iu™L «« ? ^and all outside of the egg, as an independent free-swimming larva (Fig. 109, ^). This is in sliarp^contrast to its near relative, the freshwater crayfisli, wliich goes M 2 164 THE EVOLUTION THEORY through this whole development within the egg, and emergesperfect!} formed. We see from this example that it is not some inward necessitywhich thus, in the higher and more complicated organism, contractsthe ontogeny into the embryonic state, but that this depends uponexternal adaptive factors. Here again we have adaptation, mainly to
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