HOMOLOGIES. 209 
the present Comatule and Star-Fishes with stems. 
So is it often with the study of Nature; many 
scattered links are collected before the man comes 
who sees the connection between them and 
speaks the word that reconstructs the broken 
chain. 
I will begin my comparison of all Echinoderms 
with an analysis of the Star-Fishes and Sea- 
Urchins, because I think I can best show the 
identity of parts between them, notwithstanding 
the difference in their external form; the Sea- 
Urchins having always a spherical body, while 
the Star-Fishes are always star-shaped, though in 
some the star is only hinted at, sketched out, as 
it were, in a simply pentagonal outline, while in 
others the indentations between the rays are very 
deep, and the rays themselves so intricate in their 
ramifications as to be broken up into a complete 
net-work of branches. But under all this vari- 
ety of outline, our problem remains always the 
same: to build with the same number of pieces 
a star and a sphere, having the liberty, however, 
of cutting the pieces differently and changing 
their relative proportions. Let us take first the 
Sea-Urchin and examine in detail all parts of its 
external structure. I shall say nothing of the 
internal structure of any of these animals, be- 
cause it does not affect the comparison of their 
different forms and the external arrangement 
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