192 STUDIES IN EVOLUTION 



summer eggs), show how rudimentary the nauplius append- 

 ages may become when this stage is passed within the egg. 

 Even a more marked reduction is exhibited in the embryos 

 of Palcemon and Astacus (figures 25 and 26). Cyclops is a 

 very normal form, though even here in a second nauplius 

 stage (figure 12) a fourth pair of limbs is developed. 



Examples have been cited showing the reduction and obso- 

 lescence of the anterior antennae, or first pair of nauplius 

 limbs, and some cases will now be cited in which the third 

 pair also becomes reduced and rudimentary. Achtheres 

 (figure 14) and Mysis (figure 22) afford instances of this 

 variation. The former is of additional interest, as showing 

 that the appendages from the fourth to the eighth may be 

 developed, while the third remains quiescent, and that the 

 second pair, typically biramous, is here unbranched. Simi- 

 larly, in Mysis, Nebalia (figure 19), and especially in Cypris 

 (figure 18), the nauplius limbs are simple. The embryo of 

 Lucifer (figure 24) and a late nauplius stage of Euphausia 

 (figure 21) are also of moment in showing the beginnings 

 of the metastoma (mt) with the two maxillae and first 

 maxillipedes. 



It appears from the foregoing facts that enough has been 

 shown to prove the marked variations in the number and 

 state of development of the nauplius appendages, and to 

 reach the conclusion that potentially five pairs of cephalic 

 appendages are present. The two posterior pairs are the 

 ones usually not developed until after some of the trunk 

 limbs appear. Very satisfactory explanations have been 

 offered as to why the first three pairs have been selected by 

 the larva, although it does not seem to have been recognized 

 that the fourth and fifth have been more or less suppressed 

 daring the evolution of the class. Lang ^ accounts for the 

 three pairs of nauplian limbs by saying that: "In a young 

 larva which, like the Nauplius, is hatched early from the 

 egg, only a few of the organs most necessary for independent 

 life and independent acquisition of food can be developed. 

 The 3 most anterior pairs of limbs which serve for swimming 



