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 Miiller 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 



