294 THE RECAPITULATION THEORY 



vertebrates in the position of the optic lobes, these 

 being situated at the sides instead of on the dorsal 

 surface. Development shows that this lateral 

 position is a secondarily acquired one, for through- 

 out all the earlier stages the optic lobes are, as in 

 other vertebrates, on the dorsal surface, and only 

 shift down to the sides shortly before the time of 

 hatching. 



Crabs differ markedly from their allies, the 

 lobsters, in the small size and rudimentary condi- 

 tion of their abdomen or " tail." Development 

 however affords abundant evidence of the descent 

 of crabs from macrurous ancestors, for a young crab 

 at what is termed the Megalopa stage has the 

 abdomen as large as a lobster or prawn at the same 

 stage. 



Molluscs afford excellent illustrations of recapitu- 

 lation. The typical gasteropod has a large spirally 

 coiled shell ; the limpet however has a large 

 conical shell, which in the adult gives no sign of 

 spiral twisting, although the structure of the animal 

 shows clearly its affinity to forms with spiral shells. 

 Development solves the riddle at once, telling us 

 that in its early stages the limpet embryo has 

 a spiral shell, which is lost on the formation 

 subsequently of the conical shell of the adult. 



Recapitulation is not confined to the higher 

 groups of animals, and the Protozoa themselves 

 yield most instructive examples. A very striking 

 case is that of Orbitolites, one of the most complex 

 of the procellanous Foraminifera, in which each 

 individual during its own growth and development 



