- 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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