EMBRYOLOGY AND CLASSIFICATION. 307 
suckers without a disk at their end, and to the 
various Echinoids and Holothurians, the early 
phases of whose growth are described by J. Miil- 
ler, shows plainly that the metamorphosis of the 
Comatula furnishes a scale for the classification 
of all the Crinoids of past ages, just as that of the 
common Five-Finger (Asterias) gives the key to 
the relative standing of all the families of Star- 
Fishes, the more circular or pentagonal forms of 
which are respectively inferior to their star-shaped 
allies, those with two rows of suckers inferior to 
those with four, and those with simple ambulacra 
inferior to those in which the ambulacra have 
a disk-shaped extremity. 
The beautiful investigations of Miller have 
made us acquainted with the young of several 
families of the order of Echini or Sea-Urchin, in- 
cluding the Spatangoids, so different with their 
oblong form and eccentric mouth from the cir- 
cular Sea-Urchin, with its central mouth. Yet 
the Spatangoid in its earlier stages is spheroidal, 
like the young Echinus; and the ambulacral 
apparatus, so highly differentiated in its vertical 
extension in the adult Spatangoid, is as simple in 
the young as in the Echinus. The adult Spatan- 
goid is covered with innumerable hair-like spines, 
while the young bears only a few large rods, re- 
sembling even more those of a Cidaris than those 
of an Echinus. We may, indeed, fairly say, that 
