60 INVERTEBRATE ZOOLOGY 
The nauplius larva is of great theoretical significance. It 
appears as the youngest, free-swimming larval stage of almost 
all the entomostracans and of several of the malacostracans, 
and those malacostracans which are born in a later period of 
their development pass through a nauplius stage (2.e., a stage 
in which the body is unsegmented and bears three pairs of 
appendages) while they are still in the egg. This universal 
occurrence of the nauplius larva seems to indicate that it 
repeats substantially the structure of the primitive ancestor of 
all crustaceans. 
In its further development and growth the nauplius larva 
increases in size, gradually becomes segmented, and acquires 
new appendages, its growth and the specialization of its organs 
advancing from the anterior towards the posterior end. The 
appendages, which were originally typical, unmodified crusta- 
cean appendages, become differentiated to form the first and 
second pairs of antennz and the mandibles, and finally the size 
and structure of the adult are attained. 
Exercise 2. Look for several nauplii which are somewhat 
advanced in development and draw outlines of them. 
