368 Prof. A. Agassiz on Palwontological 



It would lead me too far to institute the same comparison 

 between the embryonic stages of the different orders of 

 Echinoderms and their earliest fossil representatives. We 

 may, however, in a very general way, state that we know 

 the earliest embryonic stages of the orders of Echinoderms 

 of today, which, with the exception of the Blastoidea and 

 Cystideans, are identical with the fossil orders, and that, so 

 far as we know, they all begin at a stage where it would be 

 impossible to distinguish a Sea-urchin from a Starfish, or an 

 Ophiuran, or a Crinoid, or an Holothurian — a stage in which 

 the test, calyx, abactinal, and ambulacral systems are reduced 

 to a minimum. From this identical origin there is developed 

 at the present day, in a comparatively short period of time, 

 either a Starfish, a Sea-urchin, or a Crinoid j and if we 

 have been able successfully to compare, in the development 

 of typical structures, the embryonic stages of the young 

 Echini with their development in the fossil genera, we may 

 fairly assume that the same process is applicable when insti- 

 tuting the comparison within the different limits of the orders, 

 but with the same restrictions : that is, if we wish to form 

 some idea of the probable course of transformations which the 

 earliest Echinoderms have undergone to lead us to those of 

 the present day, we are justified in seeking for our earliest 

 representatives of the orders such Echinoderms as resemble 

 the early stages of our embryos, and in following, for them 

 as for the Echini, the modifications of typical structures. 

 These we shall have every reason to expect to find repeated 

 in the fossils of later periods ; and going back a step further 

 we may, perhaps, get an indefinite glimpse of that first 

 Echinodermal stage which should combine the structural 

 features common to all the earliest stages of Echinoderm 

 embryos. 



And yet, among the fossil Echinoderms of the oldest 

 periods, we have not as yet discovered the earliest type from 

 which we would derive either the Starfishes, Ophiurans, Sea- 

 urchins, or Holothurians. With the exception of the latter, 

 which we can leave out of the question at present, we find all 

 the orders of Echinoderms appearing at the same time. But 

 while this is the case, one of the groups attained in those 

 earliest days a prominence which it gradually loses with the 

 corresponding development of the Starfishes, Ophiurans, and 

 Sea-urchins ; it has steadily declined in importance : it is a 

 type of Crinoids, the Cystideans, which culminated during 

 Palseozoic times, and completely disappeared long before the 

 present day. If we compare the early types of Cystideans with 

 the typical embryonic Echinodermal type of the present day, 



