THE TRANSITORY NAUPLIUS STAGE. 215 



while its ends are prolonged on each side nearly as far 

 as the mouth. This is the commencement of the free 

 edge of the caraj^ace (fig. 58, E and F, and fig. 59, A, c) 

 — the lateral parts of which, greatly enlarging, become 

 the hranchiostegites (fig. 59, T>, c). 



In many animals allied to crayfish, the young, when 

 it has reached a stage in its development, which answers 

 to this, undergoes rapid changes of outward form and of 

 internal structure, without making any essential addition 

 to the number of the appendages. The appendages which 

 represent the antennules, the antennas, and the mandibles 

 elongate and become oar-like locomotive organs ; a 

 single median e3'e is developed, and the young leaves the 

 egg as an active larva, which is known as a Nauplius. 

 The crayfish, on the other hand, is wholly incapable of 

 an independent existence at this stage, and continues its 

 embryonic life within the egg case ; but it is a remark- 

 "able circumstance that the cells of the epiblast secrete 

 a delicate cuticula, which is subsequently shed. It is 

 as if the animal sjanbolized a nauplius condition by 

 the development of this cuticle, as the foetal whalebone 

 whale symbolizes a toothed condition by developing teeth 

 which are subsequently lost and. never perform any 

 function. 



In fact, in the cra3'fish, the nauplius condition is soon 

 left behind. The sternal disk spreads more and more 

 over the yelk ; as the region between the mouth and 

 the root of the abdomen elongates, slight transverse 



