386 Agassiz—Paleontological and Embryological Development. 
fully 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 pro- 
cess is applicable when instituting 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 under- 
gone 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 ex- 
pect 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 our 
Kchinoderm embryos. 
n yet, among the fossil Echinoderms of the oldest peri- 
ods, we have not as yet discovered this earliest type from which 
we could derive either the Star-fishes, 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 these earliest days a 
prominence which it gradually loses with the corresponding 
development of the Star-fishes, Ophiurans, and Sea-urchins, it 
rs the early types of Cystideans to the typical embryo- 
This may not seem a very satisfactory result to have at- 
tained. It certainly has been shown to be an impossibility to 
trace in the paleontological succession of the Echini anything 
like a sequence_of genera. No direct filiation can be shown to 
from the earliest epochs at which they occur to the present 
among the marine animals of the present day are the direct 
descendants of those of the earliest geological periods. When 
