246 DARWINISM AND THE PROBLEMS OF LIFE 



much like feet, except that the first masticators are shaped 

 like two strong teeth. But, it will be asked, how do we 

 know that these and the antennae were originally legs, 

 when they have no resemblance to legs ? 



The answer to the question is given by embryology. 

 In the case of a good many Crustacea the ovum produces 

 an animal that is very different from the adult, and is 

 known as a nauplius-larva. It is unarticulated, and has 

 a simple organisation and only three pairs of legs, with 

 which it swims about with a kind of hop. The larva 

 gradually grows ; its hind end lengthens and breaks 

 into joints, on which new limbs sprout out ; these differ 

 in number according to the number possessed by the 

 adult of the particular species. But the original three 

 pairs of limbs of the nauplius are converted into the 

 two pairs of antennae and the first masticators. These 

 have therefore been developed from legs, as we clearly 

 see. 



There are crustaceans that have become parasites ; 

 they cling to other animals, and feed on their vital 

 fluids. These have changed their form so completely 

 that they may be taken for a piece of the intestine or 

 at all events a worm, but never for a crustacean. The 

 most curious, perhaps, is the sacculina, a crustacean that 

 settles on the abdomen of a marine crab and is shaped 

 like a sac ; widely branching root-fibres proceed from 

 this and penetrate the whole interior of the crab and 

 suck its blood. Yet this organism, so little like an 

 animal, is a crustacean ; in its youth it has the form 

 of one, and it emerges from the egg as a nauplius that 

 cannot be distinguished from the nauplii of other 



