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 
egg 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 egg not so very 
unlike 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 egg in the Crustacea is called 
the Nauplius ; it develops into the Zoea, 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 Nauplius whose anatomy is as different 
from that of the mature prawn, for instance, as that of the larva 
is from the butterfly. The Nauplius has not the number of the 
extremities which characterises the Crustacea as a class, nor has 
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