THE CRUSTACEA. 449 



What appeared most unreasonable to the imperfectly informed 

 naturalist of a quarter of a century since is commending itself 

 to those who are now most qualified to judge of such questions. 

 For instance, what could be more distinct than the prawn and 

 any insect in the thoughts of the older anatomists ; the one has 

 more legs than the other, the respiratory apparatus differs, and 

 the physiology is not the same. The progress of science, and 

 the researches of Thomson, Spence Bate, Goodsir, and many 

 others, opened up the investigation by discovering and describing 

 the metamorphoses of the crab class, phenomena which were at 

 once stoutly denied. It was proved that some Crustacea left the 

 6gg totally unlike the adult form, and that a transformation 

 occurred into a creature more like the perfect animal, the pro- 

 cess of evolution being unaccompanied by any immobile state 

 like that of the chrysalis, and being assisted by repeated skin 

 sheddings. In some Crustacea the first stage was shown to be 

 absent, and that in others the animal left the c^^'g not so very 

 unUke the parent, and it was soon discovered that closely allied 

 members of the class were subject to one or other of these 

 incomplete metamorphoses, or to no transformation at all. The 

 transformation is not invariable in the evolution of the class, nor 

 is it in the true insects ; and similarity of structure, and great 

 general resemblances of shape, organisation, and habit, evidently 

 do not necessitate the same method of arriving at maturity in 

 either class. It is most probable, then, that the transformations 

 were superadded in both classes to kinds the destinies of which 

 required such phases in order to maintain the life of the species. 

 The creature which comes from the ^gg in the Crustacea is called 

 the Naiiplius ; it develops into the Zoca, and this usually turns 

 into the adult form, but there may be an intermediate stage. 

 The three phases are often as different as regards the method of 

 life and shape of the creature as are the larvae, mobile nymphs, 

 and perfect forms of such insects as the Orthoptera. 



The earliest stage of crustacean life, subsequent to the em- 

 bryonic state, presents a Nanplius whose anatomy is as different 

 from that of the mature prawn, for instance, as that of the larva 

 is from the butterfly. The Natiplius has not the number of the 

 extremities which characterises the Crustacea as a class, nor has 



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