EMBRYOLOGY AND CLASSIFICATION. 313 
cision the correspondence of all their different 
conditions. In this direction there is a boundless 
field open to the researches of young naturalists. 
In the class of Crustacea enough is already 
known to establish a correspondence between 
the young of the higher members of the class 
and the adults of its lower members; and the 
comparison may here be extended with remark- 
able precision to the fossils of past ages, since 
representatives of this class are known from the 
earliest geological epochs in which animals ex- 
isted at all to the present time. The class of 
worms has of late attracted so much attention, 
and so many of, them have been studied during 
their transformation, that, were these animals 
more generally known, I could adduce striking 
instances of this correspondence between the 
younger stages of growth in the higher mem- 
bers of the class and the adult forms of its lower 
representatives. But I will not enter into these 
details, as I have no vernacular names by which 
I could designate them intelligibly, and for pro- 
fessional naturalists this allusion is sufficient. 
They will remember that the highest worms so 
remarkable for the various locomotive and respi- 
ratory appendages on their sides, are, in their 
earlier phases, as destitute of these appendages 
as are the lowest members of the class in their 
adult condition. 
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