9° 



THE DARWINIAN THEORY 



spirally-coiled shell; the Limpet, however, has a 

 conical shell, which in the adult animal shows no 



Fig. 13. 



Young and adult specimens of one of the swimming Crabs (Portunus), to 



illustrate the transition from the long-tailed to the short-tailed conditio?: . 



A. — The Zosea stage, characterised by the great length of the spines on 

 the cephalothorax ; by the large size of the powerful rowing maxillipedes ; 

 and by the long, jointed tail. The larva is swimming in the direction of 

 the long dorsal spine, the spines serving to guide its course. x 15 



B. — The Megalopa stage. This is the typical Macrurous condition, 

 comparable to that of an adult Lobster or Prawn. x 5 



C. — The adult Crab. As compared with the Megalopa stage the 

 cephalothorax has increased greatly in width, while the tail has become 

 relatively smaller, and is carried turned forwards beneath the thorax,, x \ 



sign of twisting, although the structure of the animal 

 shows its affinity to forms with spiral shells. How- 



