808 EMBRYOLOGY AND CLASSIFICATION. 



the successive changes of the higher Echiuoids 

 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 Echinoderms, it is 

 surprising that no attention has thus far been paid 

 to the subject ; the eminent physiologist himself, 

 to whom we owe so large 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 

 landing 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, I beg leave to submit here a few state- 

 ments, the full demonstration of which may be 



