ANIMAL PEDIGREES 205 



At the megalopa stage the shape and proportions 

 are very similar to those of a lobster or other 

 macrurous decapod. It is only in the last stages 

 of development when the shape, though not the 

 size, of the adult crab is attained, that the abdomen 

 becomes relatively smaller, and is turned forwards 

 out of sight beneath the hinder part of the thorax. 



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 subse- 

 quently 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 porcellanous Foraminifera, in which each 

 individual during its own growth and development 

 passes through the series of stages by which the 

 cyclical or discoidal type of shell was derived from 

 the simpler spiral form. 



The fully formed Orbitolite shell is a thin cal- 

 careous disc. It is hollow, and the central cavity 

 is divided into chambers by concentric partitions or 

 septa. These chambers are further subdivided by 

 incomplete radial partitions. The concentric parti- 

 tions are perforated by numerous holes, which place 



