308 EMBRYOLOGY AND CLASSIFICATION. 
the successive changes of the higher Echinoids 
make us acquainted with a series of transfor- 
mations which have their counterparts, not only 
in the different families of the order as ranked 
one above the other, but also in the order of suc- 
cession of these Radiates in past geological times. 
Even among the Holothurians, imperfectly- as 
their development is known, it already appears, 
upon embryonic data, that those without external 
ambulacra are inferior to those which have them, 
since the latter are destitute of these organs in 
their earlier stages of growth. Notwithstanding 
the direct bearing of these embryological facts 
upon the classification of the Hchinoderms, it is 
surprising that no attention has thus far been paid 
to the subject; the eminent physiologist himself, 
to whom we owe so Jarge a share of our knowl- 
edge of the facts above referred to, has failed to 
perceive their significance in this connection. @ 
It would require a discussion of facts not yet 
sufficiently familiar even to naturalists, were I to 
attempt a similar comparison of the successive 
stages of growth of the Mollusks with the relative 
standing of the different members of their respec- 
tive classes; and yet, as I have by my own in- 
vestigations reached a synthesis which enables 
me to discuss the question in its most general 
bearing, 1 beg leave to submit here a few state- 
ments, the full demonstration of which may be 
