THE PROBLEMS OF TRANSFORMATION 247 



history of the common lobster ; or the newly-hatched creature 

 has its full series of limbs, wanting maybe only the hindmost 

 abdominal pair, which are acquired at the first moult, as is the 

 case with freshwater crayfishes ; or there is a remarkably 

 specialized larva adapted for floating or swimming, whence 

 the crawling adult is developed by a process of rather profound 

 change, as shown in the life-history of sea crayfish and of most 

 crabs (Fig. 120 b). -Most members of the Arachnida spiders 

 and scorpions, for example are hatched or born as miniature 



at 



an 



at 



FIG. I2O. LARVAE OF CRUSTACEA. 



a, Nauplius of copepod (ventral view) (an, antennule ; at, antenna ; in, mandible). X 100. 

 6, Zooea of crab (side view) (an, antennule ; at, antenna ; m, mandible ; m l , maxillae 

 (two pairs) ; t, first and second thoracic limbs ; the abdomen is well developed at this stage, 

 but the hinder thoracic appendages are still rudimentary). x 20. 



adults, but newly-hatched mites want the hindmost pair of 

 legs, and in the growth of these little creatures there are 

 often curiously modified stages during which migration 

 or some other crisis in the life-history can be effectually 

 got through. Millipedes are remarkable in being hatched 

 with very few segments and only three pairs of legs, the 

 long series which most of them possess when adult being 

 acquired through a course of moults. The best-known types 

 of centipede (Lithobius and its allies), with fifteen pairs of legs 

 when adult, are hatched with only seven pairs, while other 

 families of centipedes leave the egg-shell with their full 

 segmentation already developed. 



