ON THE EMBRYOLOGY OF ECHINODERMS. 25 



idea which he entertained, that we had in Echinoderms a passage from the bilateral 

 to the radiated form, should have made such a strong impression as to prevent his 

 noticing the radiated character of the young embryo, hidden as it is by all this external 

 appearance of bilateral symmetry. And had it not been for the clear idea we have 

 now of the character of the parts of radiated animals,* I doubt not that Miiller's view 

 would have gained general acceptance among investigators, and the whole frame- 

 work of classification, based upon the idea that a plan pervades the different types 

 of the animal kingdom, would have fallen to the ground, if it could have been 

 clearly proven that in Echinoderms we had a transition from one of these plans to 

 another. 



As embryology gives us the means of distinguishing on broad principles the class 

 of Batrachians from that of the true Reptiles, since it has been shown conclusively by 

 Professor Agassiz that the Batrachians are an eminently embryonic class, while the 

 Reptiles proper are a synthetic type, so the embryology of Echinoderms throws a 

 new light on the character of the orders Avhich compose that class. Particularly 

 important is this knowledge when applied to those early forms which have been 

 considered by some geologists as Starfishes, Echinoids, or Ophiurans, thus placing 

 the first appearance of these orders far back in geological times. A comparison of 

 these types with the embryonic forms of our Starfishes, Sea-urchins, and Ophiurans, 

 will show us plainly that they have nothing in common with them. The few fea- 

 tures which have misled investigators, and have prevented their recognition as true 

 Crinoids, are either synthetic or prophetic characters. Crinoids are an eminently 

 synthetic and prophetic type. From the time of the earliest appearance of Crinoids 

 the characters which they combined foreshadowed the advent of the true Starfishes, 

 the Ophiurans, and the Echinoids. The synthetic characters were so prominently 

 developed that many of them are readily mistaken for Starfishes or Sea-urchins, for 

 the same reasons which have made it so difficult to recognize as true Reptiles those 

 synthetic forms in Avhich Fish or Batrachian features concealed the true Reptilian 

 character, until we had obtained a reliable guide in the distinctions pointed out by 

 embryology. If these views are correct, the Crinoids are the only Echinoderms which 

 are found in the Palaeozoic period, and it is not until the Secondary that the other 

 orders appear. 



The Starfishes as an order are characterized by the absence of prophetic features. 

 They are rather a parembryonic order ; that is, certain features which are character- 



• L. Agassiz. Contributions to the Nat. Hist, of the U. S., Vols. III., IV. 

 VOL. IX. 4 



