HOMOLOGIES. 205 
ral law which nevertheless controls and includes 
them all. 
I wish that I could take as the illustration of 
this statement animals with whose structure the 
least scientific of my readers might be presumed 
to be familiar; but such a comparison of the 
Vertebrates, showing the identity and relation of 
structural elements throughout the Branch, or 
even in any one of its Classes, would be too ex- 
tensive and complicated, and I must resort to the 
Radiates, — that branch of the Animal Kingdom 
which, though less generally known, has the sim- 
plest structural elements. 
I will take, then, for the further illustration of 
my subject, the Radiates, and especially the class 
of Echinoderms, Star-Fishes, Sea-Urchins, and 
the like, both in the fossil and the living types; 
and though some special description of these ani- 
mals is absolutely essential, I will beg my readers 
to remember that the general idea, and not its 
special manifestations, is the thing I am aiming 
at, and that, if we analyze the special parts char- 
acteristic of these different groups, it is only that 
we may resolve them back again into the struc- 
tural plan that includes them all. 
I have already in a previous article named the 
different Orders of this Class in their relative 
rank, and have compared the standing of the liv- 
ing ones, according to the greater or less compli- 
