XII 



BACKBONELESS ANIMALS 



249 



some being sensory, others masticatorv. others locomotor, 



/ * */ ' 



and so on. The presence of two pairs of antennae on the 

 head is very characteristic. To the chitin or organic 

 foundation of the cuticle carbonate of lime is often added 

 in abundance, as is familiarly seen in the hard shells of 

 crabs and lobsters. In the higher forms the life -history 



FIG. 71. NAUPLIUS OF SACCULINA, SEEN FROM BELOW. 

 ENORMOUSLY ENLARGED. 



(From Fritz M tiller.) 



is often long and circuitous, with a succession of larval 

 stages. 



The lower Crustaceans are grouped together as Entc- 

 mostraca. They are often small and simple in structure ; 

 the number of segments and appendages varies greatly. 

 The little larva which hatches from the egg is usually a 

 " Nauplius ' an unsegmentcd creature with only three 

 pairs of appendages and a median eye. (See fig. 71.) 



The brine-shrimps (Artemia), the related genus Bran- 



