THE BIOGENETIC LAW 



163 



say, so that the nauplius stage forms a part of the embryonic develop- 

 ment, and that new segments and limbs develop in the embryo 

 nauplius within the egg, so that 

 the young animal leaves the egg- 

 in a more advanced state, nearer 

 to that of the perfect animal, to 

 which it can, therefore, attain in 

 a shorter time. 



We should expect that this 

 shortening of the larval period 

 would be associated with a pro- 

 longation of embryogenesis, es- 

 pecially in those Crustaceans 

 which possess a large number of 

 segments and limbs, that is — in 

 the higher forms — and in the 

 main this is the case. But there 

 are exceptions in two directions ; 

 in the first place there are some, 

 even among the lower Crusta- 

 ceans, which leave the egg not 

 as a nauplius but in the perfect 

 form 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 po- 

 timlrim. 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 

 through a series of higher larval 

 forms up to the perfect animal, 

 and all outside of the egg, as 

 an independent free-swimming larva (Fig. 1 09, A-E). This is in sharp 

 contrast to its near relative, the freshwater crayfish, which goes 



M 2 



Fig. 109. C, second Zoaea stage. The 

 thorax is now divided into cephalothorax 

 (Cph) and abdomen {Abd) ; seven pairs of 

 appendages are developed, and iive more 

 {VIII-XII) are beginning to appear. Av, 

 paired eyes. 



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