THE TRANSITORY NAUPLIUS STAGE. 215 
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while its ends are prolonged on each side nearly as far 
as the mouth. This is the commencement of the free 
edge of the carapace (fig. 58, E and F, and fig. 59, A, ¢) 
—the lateral parts of which, greatly enlarging, become 
the branchiostegites (fig. 59, D, 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 antenne, and the mandibles 
elongate and become oar-like locomotive organs; a 
single median eye 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 symbolized a nauplius condition by 
the development of this ctticle, 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 crayfish, 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 
