166 



THE 



PART 



primitive character of the genus. The number of 

 limbs is far in excess of the rings in the body, and if 

 we once recognise that the rings do not correspond 

 with segments except in the fully developed anterior 

 trunk region, but that each pair of limbs having its own 

 pair of ventral ganglia corresponds with a true segment 



FIG. 39. Second larval stage of Apus cancriformis (Claus), showing the gradual 

 development of Apus out of the Nauplius, the liver as diverticulaof the mid-gut. 

 /, frontal sensory organs. 



either developed or rudimentary, we have an animal, 

 say Apus cancriformis, with from 60-65 segments. 

 There are other species with from 40-50 segments. 

 In all other Crustacea the number of segments is 

 for each group either absolutely or very nearly con- 

 stant. The type is fixed. In the Apodidae, as we 



